Rule based aggregation of files and transactions in a switched file system

ABSTRACT

A switched file system, also termed a file switch, is logically positioned between client computers and file servers in a computer network. The file switch distributes user files among multiple file servers using aggregated file, transaction and directory mechanisms. The file switch distributes and aggregates the client data files in accordance with a predetermined set of aggregation rules. Each rule can be modified independently of the other rules. Different aggregation rules can be used for different types of files, thereby adapting the characteristics of the switched file system to the intended use and to the expected or historical access patterns for different data files.

RELATED APPLICATIONS

[0001] This application claims priority from U.S. patent applicationSer. No. 10/043,413, entitled File Switch and Switched File System,filed Jan. 10, 2002, and U.S. Provisional Patent Application No.60/261,153, entitled FILE SWITCH AND SWITCHED FILE SYSTEM and filed Jan.11, 2001, both of which are incorporated herein by reference.

[0002] This application is furthermore related to the followingapplications, each of which is filed on the same date as thisapplication and is hereby incorporated by reference in its entirety:Transaction Aggregation in a Switched File System, attorney docket11187-0004-999; Directory Aggregation for Files Distributed Over APlurality of Servers in a Switched File System, attorney docket11187-0005-999; Metadata Based File Switch And Switched File, attorneydocket 11187-0006-999; Aggregated Lock Management for Locking AggregatedFiles in a Switched File System, attorney docket 11187-008-999; andAggregated Opportunistic Lock and Aggregated Implicit Lock Managementfor Locking Aggregated Files in a Switched File System, attorney docket11187-009-999.

FIELD OF THE INVENTION

[0003] The present invention relates generally to the field of storagenetworks, and more specifically to file switching and switched filesystems.

DESCRIPTION OF THE REALTED ART

[0004] Since the birth of computer networking, access to storage hasremained among the most important network applications. The reason issimple: the purpose of networks was and is to share data and content,and most of the data worth sharing resides on some form of storage.

[0005] Despite the importance of storage applications in networks, theirusefulness has, until recently, been greatly limited by the insufficientbandwidth provided by networks. Even at 100 Megabits/second (Mbps) (themost common maximum speed in existing local area networks, also known asFast Ethernet), accessing data through a network is several times slowerthan reading it from a hard disk attached locally to a computer. Forthis reason, historically most of the data accessed by a networkedcomputer (workstation or application server—often referred to as a“client”) has resided on local storage and only data that has to beshared has resided on network servers.

[0006] The introduction of gigabit and multi-gigabit network technology,however, is changing the rules of the game. A single Gigabit Ethernet orFibreChannel connection is capable of transporting data at aggregaterates of up to 240 Megabytes/second (MB/s), which is much greater thanthe performance of most locally attached storage devices. This meansthat in new high speed networks, data can be accessed through thenetwork faster than from local storage. As a result, we have now reachedthe beginning of a fundamental trend in which the majority of usefuldata is being moved to the network.

[0007] Storage Networks

[0008] The ability to store terabytes of data on the network and makethat data accessible to tens and hundreds of thousands of users isextremely attractive. At the same time, creating storage and networksystems capable of adequately handling such amounts of information andusage loads is not a simple task. As a result, storage networking—thediscipline that deals with designing, building and managing suchsystems—is rapidly becoming recognized as a separate, specialized fieldof computer networking.

[0009] The key promise of storage networking is in delivering networksystems that enable the sharing of huge amounts of information andcontent among geographically dispersed users. To deliver on thispromise, the storage network systems have to be extremely scalable whileproviding a high degree of availability comparable to that of the publictelephone system. In addition, any system of this scale has to bedesigned so that it can be managed effectively.

[0010] Available Approaches to Scaling File Systems

[0011] The primary function of every file system is to enable sharedaccess to storage resources. In fact, file systems were originallycreated to facilitate sharing of then-expensive storage between multipleapplications and multiple users. As a result, when exposed as a networkservice, file systems provide a complete and mature solution to theproblem of sharing data.

[0012] The flip side is that file systems are complex and veryprocessing-intensive, which increases substantially the performancerequirements to any computer that provides file services over a fastnetwork. To serve files to hundreds and thousands of userssimultaneously requires tremendous amounts of processing power, memoryand bus bandwidth.

[0013]FIG. 1 illustrates a typical application of presently available,commonly used network file systems. The system consists of a local areanetwork 104, which connects a large number of client workstations andapplication servers 102, connected to various file servers. The fileservers typically include standalone servers such as 105 and 106, aswell as file servers, such as 107 and 108, configured as a cluster 110with shared storage 118. The servers 107 and 108 are connected togetherthrough a high-speed, low-latency intra-cluster connection 112, and arealso connected to the shared storage 118 through a SAN (storage areanetwork), typically using optical (FibreChannel) interconnect 114 and116. In addition, clients and application servers 102 and file servers105 through 108 may be configured to be part of a distributed filesystem with the appropriate software services installed on all of thosemachines.

[0014] Single Box Solutions

[0015] Single box solutions provide a simple and straightforwardapproach to the problem of increasing the performance of file servers.Traditionally, the fastest available computers were used to serve files;when even these became insufficient, specialized architectures werebuilt to extend the capabilities of the server. Where one processor wasnot enough, more processors were added; where the bandwidth of astandard bus was not sufficient, additional busses or evencustom-designed wider busses were introduced, and so on.

[0016] The result of this approach is that high-end file servers areessentially massively multiprocessing supercomputers, with all theassociated costs and complexity. Examples of single box solutions arethe EMC Celera/Symmetrix, SGI Origin, HP Superdome, Intel Paragon andIBM SP, the trademarks of which are hereby acknowledged. However,high-performance multiprocessing file servers quickly run into theperformance limits of their storage subsystems. The approach toresolving this bottleneck is to spread the load among multiple harddisks and data paths operating in parallel.

[0017] Single-box solutions are subject to several serious problems.First, because of the extremely high complexity and the need to developcustom silicon in order to satisfy performance requirements, single boxsolutions are very expensive. Second, their development cycles areexceedingly long, virtually guaranteeing that they will be “behind thecurve” in many important aspects, such as software technologies,protocols, etc., by the time they are generally commercially available.Since storage requirements effectively double every year or so, theseboxes often become obsolete long before the customers manage todepreciate their high cost.

[0018] Cluster File Systems

[0019] An alternative to scaling the server architecture within the boxis to put together multiple servers accessing the same pool of storageover a fast interconnect such as HIPPI or FibreChannel. The result is a“cluster” of computers that acts in many aspects similarly to amultiprocessing supercomputer but can be assembled from generallyavailable components.

[0020] Since all computers in a cluster access the same set of harddisks, the file system software in each of them has to cooperate withthe other members of the cluster in coordinating the access andallocation of the storage space. The simplest way to approach thisproblem is to section the storage pool and divide it among the differentcomputers in the cluster; this approach is implemented in Windowsclustering described in “Windows Clustering Technologies—An Overview”,November 2000, Microsoft Corp. The main challenge in the above-mentionedfile system comes from the need to frequently synchronize and coordinateaccess to the storage among all members of the cluster. This requires acentralized lock manager and/or a file manager that controls theallocation of disk space to different files and controls access to thosefiles. These components quickly become a major bottleneck that preventsthe scaling of cluster file systems beyond about sixteen nodes.

[0021] The reliance on centralized resource coordination is the primaryweak point of cluster file systems that limits severely theirscalability. Solutions that partially relieve this problem introduceother problems, including custom functionality in storage subsystems andspecialized client-side software. If any of these approaches iscommercialized, the requirement for using proprietary storage subsystemswill have substantial negative effect on both adoption and price, whilethe need to rely on proprietary client-side software that has to beinstalled in every client accessing the system make the system fragile,prone to security breaches and hard to deploy and support.

[0022] Distributed File Systems

[0023] Both single box solutions and cluster file systems are tightlycoupled systems that exhibit serious scalability limitations. Creatingdistributed file systems is an approach attempting to combine hundredsof file servers in a unified system that can be accessed and managed asa single file system. Examples of distributed file systems are theAndrew File System, and its derivatives AFS and Coda, Tricord, as wellas the Microsoft Distributed File System DFS.

[0024] Distributed file systems are loosely coupled collections of fileservers that can be located in diverse geographical locations. Theyprovide a unified view of the file namespace, allowing clients to accessfiles without regard to where in the system those files reside. Inaddition, the system administrator can move files from one server toanother in a transparent fashion and replicate files across multipleservers for increased availability in case of partial system failure.

[0025] Distributed file systems exhibit excellent scalability in termsof storage capacity. It is easy to add new servers to an existing systemwithout bringing it off-line. In addition, distributed file systems makeit possible to connect storage residing in different geographicallocations into a single cohesive system.

[0026] The main problem with available distributed file systems is thatthey do not scale in performance nearly as well as they scale in storagecapacity. No matter how large the number of servers in the system, eachindividual file resides on exactly one server. Thus, the performance thedistributed file system can deliver to a single client (workstation orapplication server) is limited by the performance of the utilizedindividual file servers, which, considering the large number of serversinvolved, is not likely to be a very high performance machine.

[0027] Another problem that has great impact in commercial environmentsis the fact that most distributed file systems require specializedclient-side software that has to be installed and configured properly oneach and every client that is to access the file system. This tends tocreate massive versioning and support problems.

[0028] Moreover, distributed file systems are very prone to“hotspotting”. Hotspotting occurs when the demand for an individual fileor a small set of files residing on a single server increasesdramatically over short period of time, resulting in severe degradationof performance experienced by a large number of users.

[0029] Yet another problem with distributed file systems is in their lowmanageability. Although most aspects of the distributed file systems canbe managed while the system is on-line, the heterogeneous anddistributed nature of these systems effectively precludes any seriousautomation of the management tasks. As a result, managing distributedfile systems requires large amount of highly qualified labor.

[0030] Summary

[0031] Although many approaches to scaling network file systems havebeen taken over the last fifteen years, none has succeeded in deliveringon the high performance, high scalability and simple management promiseof storage networking. Analysis of the systems described above showsthat all of their limitations can be traced to a small set offundamental flaws, namely, all available systems suffer from at leastone of the following problems:

[0032] 1. One file, one server. The inability to utilize multiple fileservers in handling requests for a single file limits severely thethroughput available to any single client and does not allow the systemto balance the load across all available processing resources.

[0033] 2. Centralized arbitration and metadata management. The need toarbitrate access to storage and the shared data structures used tomanage it creates a bottleneck that severely limits the scalability ofthe system.

[0034] 3. Proprietary client-side software. The need to buy, install,configure and support a non-trivial piece of software across all clientmachines running multiple different operating systems creates seriousbarrier for adoption.

[0035] Conclusions

[0036] With the mass adoption of gigabit and multi-gigabit networkinfrastructure, storage networking is rapidly becoming key to deliveringand managing content on the network. To achieve this, storage networkshave to facilitate sharing of data among thousands (or even largernumbers) of users, be able to scale in storage capacity, performance andaccess bandwidth extremely well, provide a very high degree ofavailability, and be easy to manage. Increasingly, new applications,such as e-mail, streaming video content, document repositories, andother soft-structured data, require these characteristics to be achievedby a network service that provides access to files.

[0037] The existing approaches to scaling network file systems aresuccessful in solving one or another aspect of these requirements.However, there is no currently available system that can deliver allcharacteristics needed for storage networking to achieve its promise.

SUMMARY OF THE INVENTION

[0038] A switched file system, also termed a file switch, is logicallypositioned between client computers and file servers in a computernetwork. The file switch distributes user files among multiple fileservers using aggregated file, transaction and directory mechanisms. Ahierarchical directory structure of data files is created on the fileservers, with each data file that stores a portion of a user file havinga global unique identifier that determines at least a portion of thedata file's file path with the hierarchical directory structure. Ahierarchical directory structure on the file servers is also used tostore metadata files, which store metadata for each user file toindicate where the data files for the user file are stored. The fileswitch automatically and randomly spreads the data files and metadatafiles over a large number of distinct directories on multiple fileservers, preventing large number of data files from being stored in asingle directory on a single file server. Hence, the file switchbalances the load and improves the performance of the switched filesystem. In response to a directory enumeration request from a clientcomputer, one or more directories of metadata files on one or more ofthe file servers is enumerated, instead of enumerating the data filethat store the user file portions.

[0039] The file switch distributes and aggregates the client data filesin accordance with a predetermined set of aggregation rules. Each rulecan be modified independently of the other rules. Different aggregationrules can be used for different types of files, thereby adapting thecharacteristics of the switched file system to the intended use and tothe expected or historical access patterns for different data files.Also, multiple file switches communicate with the same set of fileservers without interacting with each other, thereby making it possibleto scale access bandwidth of the switched file system by adding fileswitches whenever needed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The aforementioned features and advantages of the invention aswell as additional features and advantages thereof will be more clearlyunderstood hereinafter as a result of a detailed description of apreferred embodiment of the invention when taken in conjunction with thefollowing drawings in which:

[0041]FIG. 1 illustrates a prior art storage network including adistributed file system and a clustered file system;

[0042]FIG. 2 illustrates a file switch in a computer network;

[0043]FIG. 3 illustrates a switched file system;

[0044]FIG. 4 illustrates transaction aggregation by a file switch;

[0045]FIG. 5 illustrates the client's view of a switched file system;

[0046]FIG. 6 illustrates the hardware architecture and memory structureof a file switch;

[0047]FIG. 7 illustrates the data plane of a file switch;

[0048]FIG. 8 illustrates an exemplary metafile;

[0049]FIG. 9 illustrates namespace aggregation by a file switch;

[0050]FIG. 10 illustrates data aggregation through mirroring;

[0051]FIG. 11 illustrates data aggregation through striping;

[0052]FIG. 12 illustrates data aggregation through spillover;

[0053]FIG. 13 illustrates the syntax of data aggregation rules;

[0054]FIG. 14 illustrates a method for creating directory structure fora metafile;

[0055]FIG. 15 illustrates the storage of metafile and user file;

[0056]FIG. 16 illustrates a method for creating directory structure fora data stream file;

[0057]FIG. 17 illustrates a method for creating directory path withglobal unique identifier;

[0058]FIG. 18 illustrates a method for balancing load at the file switchlevel;

[0059]FIG. 19 illustrates a method for transaction aggregation;

[0060]FIG. 20 illustrates a method for accessing an aggregated user filethrough the metafile;

[0061]FIG. 21 illustrates an exemplary concurrency problem;

[0062]FIG. 22 illustrates a method for implementing an implicit lockingmechanism;

[0063]FIG. 23a illustrates a method for handling an opportunity lockingrequest;

[0064]FIG. 23b illustrates a method for handling an opportunity lockingbreak notification;

[0065]FIG. 23c illustrates a method for mapping level of exclusivity ofcaching to the oplock exclusivity level granted;

[0066]FIG. 24 illustrates a method for handling a semaphore lockingmechanism;

[0067]FIG. 25 illustrates a method for enumerating a directory;

[0068]FIG. 26 illustrates a method for implementing a redundantmetavolume controller.

DETAILED DESCRIPTION

[0069] The following description is provided to enable any personskilled in the art to which the invention pertains to make and use theinvention and sets forth the best modes presently contemplated by theinventor for carrying out the invention. Various modifications, however,will remain readily apparent to those skilled in the art, since thebasic principles of the present invention have been defined hereinspecifically to provide a file switch, a switched file system and theirmechanisms of operation. Any and all such modifications, equivalents andalternatives are intended to fall within the spirit and scope of thepresently claimed invention.

[0070] Definitions

[0071] Aggregator. An “aggregator” is a file switch that performs thefunction of directory, data or namespace aggregation of a client datafile over a file array.

[0072] Data Stream. A “data stream” is a segment of a stripe-mirrorinstance of a user file. If a data file has no spillover, the first datastream is the stripe-mirror instance of the data file. But if a datafile has spillovers, the stripe-mirror instance consists of multipledata streams, each data stream having metadata containing a pointerpointing to the next data stream. The metadata file for a user filecontains an array of pointers pointing to a descriptor of eachstripe-mirror instance; and the descriptor of each stripe-mirrorinstance in turn contains a pointer pointing to the first element of anarray of data streams.

[0073] File Array. A “file array” consists of a subset of servers of aNAS array that are used to store a particular data file.

[0074] File Switch. A “file switch” performs file aggregation,transaction aggregation and directory aggregation functions, and islogically positioned between a client and a set of file servers. Toclient devices, the file switch appears to be a file server havingenormous storage capabilities and high throughput. To the file servers,the file switch appears to be a client. The file switch directs thestorage of individual user files over multiple file servers, usingstriping to improve throughput and using mirroring to improve faulttolerance as well as throughput. The aggregation functions of the fileswitch are done in a manner that is transparent to client devices.

[0075] Switched File System. A “switched file system” is defined as anetwork including one or more file switches and one or more fileservers. The switched file system is a file system since it exposesfiles as a method for sharing disk storage. The switched file system isa network file system, since it provides network file system servicesthrough a network file protocol—the file switches act as network fileservers and the group of file switches may appear to the clientcomputers as a single file server.

[0076] Data File. In the present invention, a file has two distinctsections, namely a “metadata file” and a “data file”. The “data file” isthe actual data that is read and written by the clients of a fileswitch. A file is the main component of a file system. A file is acollection of information that is used by a computer. There are manydifferent types of files that are used for many different purposes,mostly for storing vast amounts of data (i.e., database files, musicfiles, MPEGs, videos). There are also types of files that containapplications and programs used by computer operators as well as specificfile formats used by different applications. Files range in size from afew bytes to many gigabytes and may contain any type of data. Formally,a file is a called a stream of bytes (or a data stream) residing on afile system. A file is always referred to by its name within a filesystem.

[0077] Metadata File. A “metadata file”, also referred as the“metafile”, is a file that contains the metadata, or at least a portionof the metadata, for a specific file. The properties and stateinformation about a specific file is called metadata. In the presentinvention, ordinary clients cannot read or write the content of themetadata files, but still have access to ordinary directory information.In fact, the existence of the metadata files is transparent to theclients, who need not have any knowledge of the metadata files.

[0078] Mirror. A “mirror” is a copy of a file. When a file is configuredto have two mirrors, that means there are two copies of the file.

[0079] Network Attached Storage Array. A “Network Attached Storage (NAS)array” is a group of storage servers that are connected to each othervia a computer network. A file server or storage server is a networkserver that provides file storage services to client computers. Theservices provided by the file servers typically includes a full set ofservices (such as file creation, file deletion, file access control(lock management services), etc.) provided using a predefined industrystandard network file protocol, such as NFS, CIFS or the like.

[0080] Oplock. An oplock, also called an “opportunistic lock” is amechanism for allowing the data in a file to be cached, typically by theuser (or client) of the file. Unlike a regular lock on a file, an oplockon behalf of a first client is automatically broken whenever a secondclient attempts to access the file in a manner inconsistent with theoplock obtained by the first client. Thus, an oplock does not actuallyprovide exclusive access to a file; rather it provides a mechanism fordetecting when access to a file changes from exclusive to shared, andfor writing cached data back to the file (if necessary) before enablingshared access to the file.

[0081] Spillover. A “spillover” file is a data file (also called a datastream file) that is created when the data file being used to store astripe overflows the available storage on a first file server. In thissituation, a spillover file is created on a second file server to storethe remainder of the stripe. In the unlikely case that a spillover fileoverflows the available storage of the second file server, yet anotherspillover file is created on a third file server to store the remainderof the stripe. Thus, the content of a stripe may be stored in a seriesof data files, and the second through the last of these data files arecalled spillover files.

[0082] Strip. A “strip” is a portion or a fragment of the data in a userfile, and typically has a specified maximum size, such as 32 Kbytes, oreven 32 Mbytes. Each strip is contained within a stripe, which is a datafile containing one or more strips of the user file. When the amount ofdata to be stored in a strip exceeds the strip's maximum size, anadditional strip is created. The new strip is typically stored in adifferent stripe than the preceding stripe, unless the user file isconfigured (by a corresponding aggregation rule) not to be striped.

[0083] Stripe. A “stripe” is a portion of a user file. In some cases anentire file will be contained in a single stripe. Each stripe is (or isstored in) a separate data file, and is stored separately from the otherstripes of a data file. As described elsewhere in this document, if thedata file (also called a “data stream file”) for a stripe overflows theavailable storage on a file server, a “spillover” file is created tostore the remainder of the stripe. Thus, a stripe is a logical entity,comprising a specific portion of a user file, that is distinct from thedata file (also called a data stream file) or data files that are usedto store the stripe.

[0084] Stripe-Mirror Instance. A “stripe-mirror instance” is an instance(i.e., a copy) of a data file that contains a portion of a user file ona particular file server. There is one distinct stripe-mirror instancefor each stripe-mirror combination of the user file. For example, if auser file has ten stripes and two mirrors, there will be twenty distinctstripe-mirror instances for that file. For files that are not striped,each stripe-mirror instance contains a complete copy of the user file.

[0085] Subset. A subset is a portion of thing, and may include all ofthe thing. Thus a subset of a file may include a portion of the filethat is less than the entire file, or is may include the entire file.

[0086] User File. A “user file” is the file or file object that a clientcomputer works with, and is also herein called the “aggregated file.” Auser file may be divided into portions and stored in multiple data filesby the switched file system of the present invention.

File Switch and Switched File System

[0087]FIG. 2 illustrates an inventive network configuration including afile switch 200. In this configuration, the file switch 200 isimplemented with two different network interfaces: one for connecting tothe client network 211 through connection 209, and the other forconnecting to a file server network through connections 210 and othersimilar connections as shown. For simplicity, the file switch 200 isshown in this Figure as being directly connected to each of the fileservers 201 through 207. In practice, one or more commonly availablelayer 2 switches are preferably used to implement these connections.

[0088] Since most popular network file protocols are based on the IPstandard, the file switch preferably supports TCP/IP network protocols,as well as other protocols of the IP stack (e.g., ARP), as appropriate.The file switch preferably supports multiple industry standard networkfile protocols, such as NFS and CIFS.

[0089] Clients, such as workstations and application servers 212 requestfile services by communicating to the file switch 200 using the NFS orCIFS protocols. File switch 200 preferably implements the server side ofthe appropriate network file protocol on the connection 209. The switchfurther interacts with the file servers 201 through 207 by implementingthe client side of preferably the same network file protocol. Thepresence of file switch 200 is thereby preferably transparent to boththe clients and the servers.

[0090] Additionally, the file switch may implement other IP protocols,such as DHCP, DNS or WINS, either as a client or as a server for purposeof configuring file servers 201 through 207, self-configuration of thefile switch, and others that will be described herein.

[0091] The file switch 200 implements industry standard protocols bothon the client side (via connection 209) and on the server side (viaconnections 210). This implementation allows the file switch 200 tofunction in an environment where the file servers 201 through 207 arestandard, commercially available file servers or NAS appliances, andclients 212 are standard commercially available computers. In thismanner, the benefits of the file switch can be utilized withoutrequiring any proprietary software to be installed and maintained on anyother network node.

[0092] The primary functionality of the file switch can be divided intothree broad categories: 1) transaction handling; 2) file systemaggregation; and 3) switch aggregation. Transaction handling includestransaction switching and transaction aggregation. File systemaggregation includes aggregating file system objects and data file.Switch aggregation includes various mechanisms for combining multiplefile switches together, which includes load balancing, configurationsharing, fail-over and management aggregation. The functionality of thefile switch may be implemented in software, in hardware or anycombination of software and hardware, as appropriate.

[0093] A switched file system is a distributed file system as itaggregates the namespaces of multiple file servers. It is also aparallel file system, as it can utilize multiple file servers inparallel to satisfy the request of a single network file client.Therefore, the switched file system is a new type of distributed,parallel network file system.

[0094]FIG. 3 illustrates a switched file system, including itsconfigurations and applications. The exemplary switched file systemconsists of the following elements. A set of file switches 308 areaggregated in a group 309, and are connected to two arrays of fileservers 310 and 311, which are called NAS arrays. The file switches 308are also connected to a legacy file server 313, typically containingarchive and other pre-file switch content, which is aggregated only bynamespace (i.e., the file switches 308 do not perform file aggregationfor the files stored by the legacy file server 313). In addition, thefile switch group 309 aggregates the namespace of another switched filesystem provided by the file switch group 314 connected to NAS array 315and connected to the group 309 through a layer 2 switch 312.

[0095] The services of the group 309 are provided to a network 305 thatincludes clients 306, a management workstation 307 and a connection to ametro-area network 304. The metro-area network 304 provides the remoteLAN 300 and its clients 301 with file services made available by group309. In order to improve the access to these services, the remote LAN300 also includes a file switch 302, which acts as a gateway to thegroup 309 and caches files locally to the NAS array 303.

[0096] Topologies

[0097] The switched file system provides many combinations of filesystem aggregation and supports different topologies.

[0098] One of the available topologies is virtualization. Invirtualization, the switched file system aggregates the namespaceexposed by a single file server (e.g., legacy file server 313) withoutfurther aggregating its files on other servers. One of the mechanismsavailable for this is the namespace aggregation technique describedherein. The virtualization allows pre-existing file servers to be madeavailable to clients of the switched file system and included in itslogical namespace. This functionality facilitates the adoption of theswitched file system and provides an incremental approach to adoption.

[0099] Another available topology is NAS array. The switched file systemcan have a set of file servers (e.g., the servers in array 310),preferably with similar capacity and performance characteristics,designated as a NAS array. The file switches participating in theswitched file system distribute files across the file servers in the NASarray, by using the directory, and data aggregation mechanisms describedherein. NAS arrays provide high performance and high availability.Multiple NAS arrays can be configured in the same switched file system,and their namespaces can be aggregated with virtualized file servers topresent a unified namespace.

[0100] Yet another available topology is cascading. In a cascadedconfiguration, one or more switched file systems can be connected withinanother switched file system, effectively playing the role of a fileserver in that other switched file system. In our example, the fileswitches 314 and the NAS array 315 comprise a small switched filesystem, which is aggregated in the namespace of the switched file systempresented by the group 309. Since the file switches 314 appear as a fileserver to the file switches 309, the latter can aggregate the namespaceprovided by the former the same way as the virtualized server 313. Oneskilled in the art will easily recognize that multiple instances of theswitched file system comprising the file switches 314 and the NAS array315 may exist, and may be aggregated by the switches in the group 309 inany and all ways in which the latter may aggregate regular file servers,including data aggregation, directory aggregation, and so on.

[0101] Another topology is the gateway topology. A file switch 302,preferably having its own NAS array 303, acts as a gateway to clientslocally connected to it, and provides access to the file services madeavailable by the file switch group 309. An advantage of this topology isthat the connection between group 309 and file switch 302, such as theMAN 304, may have lower bandwidth than the local networks 305. Thegateway topology allows the gateway file switch 302 to cache locally onthe NAS array 303 files normally residing on the file system exposed bythe group 309. Since the file switch 302 appears as just another clientto the file switch group 309, all locking and other client semantics areavailable to the file switch 302 to provide caching.

[0102] Basics of Transaction Aggregation By a File Switch

[0103] The typical operation of the file switch involves receiving fileprotocol requests, such as login, tree connect/mount, file open, fileread/write, etc., from clients and forwarding, or switching theserequests to one or more of the file servers.

[0104]FIG. 4 illustrates a preferred process by which a file switch candelegate a single transaction received from a client to more than onefile server and therefore aggregate the behavior of those servers inhandling the transaction. The behavior of the file switch is presentedto the original client as the behavior of a single file server.

[0105] Consider the case in which a file switch 400 stripes the data ofa file among file server 401, connected to the file switch throughconnection 403, and file server 402, connected to the file switchthrough connection 404, in order to deliver higher aggregate performanceto clients by making these two file servers handle requests in parallel.

[0106] In this example, a client 406 is connected through a computernetwork 407 to the file switch 400 through connection 408. The client406 has established preferably a TCP connection to the file switch 400,and believes the file switch 400 to be a file server. The client 406,therefore, initiates a file write transaction of a file named myFile.docby issuing a write request message to the file switch 400. Afterreceiving the write request message, the file switch is in a position todecide how to handle the transaction.

[0107] In this example, the switch handles the transaction by splittingit into two transactions targeted to two separate file servers 401 and402. Upon examining the write request, the file switch updates its state(as discussed in more detail below) in a manner sufficient to accomplishthe goal, and forwards the write request to the file servers 401 and 402via the connections 403 and 404, respectively. The two file servers 401and 402 receive separate file write requests, each for its appropriatefile and each with the appropriate portion of the data to be written.The file servers execute the requested write operations in parallel andsubmit their respective responses to the file switch, which they believeto be the originator of the write requests. It should be noted that thisprocess does not require in any way that servers 401 and 402 interactwith one another or even be aware of the other's existence.

[0108] Upon receipt of responses from file servers 401 and 402,respectively, the file switch 400 knows the results of both writerequests submitted by it and is, therefore, in a position to form aresponse to the original client containing the aggregate result of thetransaction. The switch achieves this by sending an acknowledgement tothe original client. The client receives the response and sends the filemyFile.doc to the file switch. The file switch in turn sends the filemyFile.doc to the appropriate directory in servers 401 and 402. Thetransaction is now complete.

[0109] The mechanism described above enables two innovative results.First, the file switch can aggregate a set of file system entities, suchas files or directories that reside on different file servers andpresent this set to the clients as a single cohesive entity, therebyforming the foundation for aggregating complete file systems.

[0110] Second, this mechanism allows the switch to split or replicateindividual read and write network file transactions among multiple fileservers, which execute the requested operations in parallel. In thismanner, the present invention sets the foundation for forming theequivalent of a parallel file system on a network including fileswitches and file servers. The file switch has the ability to deliveraggregate performance to each client that is many times higher than theperformance of the individual file servers available to it.

[0111] Client's View of the Switched File System

[0112] From the standpoint of a network file client, such as 406, theswitched file system appears as a single file server with multiplenetwork interfaces. FIG. 5 illustrates the similarity between a switchedfile system and a single file server. Network clients connect to theswitched file system 500 through the interfaces 501 as they wouldconnect to the single file server 502 though its interfaces 503.

[0113] The switched file system 500 preferably provides a singlenamespace. It allows network file clients to use standard clientsoftware using widely standardized network file protocols for accessingfile servers, such as the CIFS and NFS protocols. The ability ofstandard file client software to access the switched file systemsimplifies adoption and also allows changes to the switched file systemmechanisms and topologies to be performed transparently to all clients.

[0114] Administrator's View of the Switched File System

[0115] An administrator's view of the switched file system 500 is to adegree similar to the client's view. For most operations, theadministrator views the switched file system 500 as if it were a single,high-capacity, high-performance, and highly available file server 502.For the purposes of management and reconfiguration it preferably appearsas a single file server. The file switches preferably support the samefile server management protocols (such as MSRAP) as single CIFS or NFSfile servers do. The switched file system can be configured to exposeshares/mount points in the aggregated namespace to their clients.

[0116] Administrators can add individual file servers (using thevirtualization topology) and new NAS arrays to the switched file system500, and can also add or remove file servers to or from existing NASarrays in the switched file system. In the event the administrator addsone or more file servers to an existing NAS array, the file switch candiscover the newly added servers (or automatically have access to theadded servers). And preferably on administrator's request, the fileswitches redistribute the files and their data across all file servers,including the newly added ones, thus extending both the capacity and theperformance of the file system. In case the administrator wishes toremove one or more file servers from a NAS array, the administrator canrequest that a file switch free up specified servers (by redistributingthe files to the file servers that remain in the NAS array). Uponcompletion of that process, the file switches notifies the administratorthat the selected file servers are free and can be removed without dataloss.

[0117] The switched file system 500 provides high availability bydistributing the work among many file switches and file servers. Failureof a file server or a file switch typically does not cause loss of dataor loss of access. The administrator can be notified of the failure andreplace or repair the failed component.

[0118] The switched file system preferably tracks access patterns andcan report statistical information to the administrator. Based on thisinformation, the administrator can tune the performance and storagecapacity utilization of the switched file system 500, for instance byadding or reconfiguring NAS arrays, file switches and by changingaggregation rules (discussed below) on the file switches.

[0119] Scaling in Switched File System

[0120] The switched file system scales capacity and performance byadding more file servers to a NAS array and distributing files acrossall file servers. It scales access bandwidth by adding more fileswitches to a connected group and accesses the same set of file servers,providing a wider access path (multiple network connections). Unlikeprior art solutions, the switched file system scales independently inmultiple directions (or dimensions) without inherent limitations.

[0121] The switched file system also scales in geographical distributionby adding cascaded file switches (or switched file system) and gatewayfile switches.

Metadata Based Switched File System

[0122] Hardware Architecture

[0123] In a preferred embodiment, each file switch 400 (FIG. 4) of themetadata based switched file system is implemented using a computersystem schematically shown in FIG. 6. The computer system (i.e., thefile switch) one or more processing units (CPU's) 600, at least onenetwork or other communications interface 604, a switch 603 or businterface for connecting the network interfaces to the system busses601, a memory device 608, and one or more communication busses 601 forinterconnecting these components. The file switch may optionally have auser interface 602, although in some embodiments the file switch ismanaged using a workstation connected to the file switch via one of thenetwork interfaces 604. In alternate embodiments, much of thefinctionality of the file switch may be implemented in one or moreapplication specific integrated circuits (ASIC's), thereby eithereliminating the need for a CPU, or reducing the role of the CPU in thehandling file access requests by client computers.

[0124] The memory 608 may include high speed random access memory andmay also include non-volatile memory, such as one or more magnetic diskstorage devices. The memory 608 may include mass storage that isremotely located from the central processing unit(s) 600. The memory 608preferably stores:

[0125] an operating system 610 that includes procedures for handlingvarious basic system services and for performing hardware dependenttasks;

[0126] a network communication module 611 that is used for controllingthe communication between the system and various clients 606 and fileservers via the network interface(s) 604 and one or more communicationnetworks, such as the Internet, other wide are networks, local areanetworks, metropolitan area networks, and so on;

[0127] a file switch module 612, for implementing many of the mainaspects of the present invention;

[0128] state information 620, including transaction state 621, open filestate 622 and locking state 623; and

[0129] cached information 624, including cached (and aggregated) datafile 626 and corresponding metadata files 625.

[0130] The file switch module 612, the state information 620 and thecached information 624 may include executable procedures, sub-modules,tables and other data structures.

[0131] In other embodiments, additional or different modules and datastructures may be used, and some of the modules and/or data structureslisted above may not be used.

[0132] Software Architecture

[0133] Layering Model

[0134]FIG. 6 also illustrates the preferred software architecture for ametadata based switched file system. The software architecture of theswitched file system is preferably divided in three planes: the coreservices plane 613, the control plane 614, and the data plane 615.

[0135] The core services layer 613 provides basic services to allcomponents in the remaining layers. These services include servicesprovided by the operating system (memory management, component model,threading), as well as services developed specifically for the fileswitch as an unattended and always-on device (configuration database,event manager, etc.). These services are general, low-level computerservices, and are minimally dependent on the particular functions of afile switch.

[0136] The control plane layer 614 is responsible for maintaining theoperation of the data plane 615. It sets up the configuration of thedata plane, controls the life cycle of the file switch, such as start,stop, and restart, and implements various management protocols. Inaddition, it includes additional services that provide features likeclustering of file switches, load balancing, failover, backup, filesystem check and repair, and automated management. These functions don'tparticipate directly in serving client-originated file requests, but areessential for the existence and continued operation of the file switch.These functions may also include value-adding services, such as datamigration and accounting.

[0137] The data plane layer 615 is responsible for file switching andaggregation. It provides all protocol layers through which file requestspass as well as the switching logic that distributes these requests tothe file servers and aggregates the responses. All requests to accessfiles and user file directories go through the data plane 615 and areserved by it.

[0138] The Data Plane

[0139] In the preferred embodiment illustrated in FIG. 7, the data planeconsists of the following key components.

[0140] The TCP/IP Transport 708 includes the NetBT (NETBIOS over TCP/IP)sub-layer used by the Server Service (SRV) 718 and Parallel Redirector706 (RDR) components. This includes the entire transport layer from theTCP or NetBT session layer down to the physical Ethernet interface. Forfast operation and minimum load on the CPU, the file switch uses ahardware-implemented or hardware-assisted extension of the TCP/IPimplementation. However, the use of hardware-assisted TCP is notrequired for the file switch to operate because the components thatinterface with TCP, such as SRV 718 and RDR 706, use the standardtransport protocol interface provided by the TCP/IP transport.

[0141] The Server Service 718 (SRV) is the CIFS file server service. Itinterprets the clients'requests for operations on files sent as CIFScommands and translates them to NT/WDM file I/O requests (IRPs). SRV 718handles the entire process of authenticating clients. Other fileprotocol servers can be used instead of or along with the CIFS fileserver (e.g., NFS).

[0142] The Virtual File System 702 (VFS) is a file system driver, anInstallable File System, in WDM terms. VFS 702 provides the common namespace of the File Switch, which makes multiple NAS Arrays combined intoaggregated file systems along with legacy single-server NAS file systemsappear as a single file system to the client. In addition, VFS serves asa “security context gateway”, working in the context of the connectedclient on its front side and providing the mandated access controlchecks, while operating in the “local system” context when accessing theconstituent file systems that make up the “virtual” namespace. Finally,VFS implements the local caching of open files to provide low latency tothe clients and optimize access to the constituent server file systemsby consolidating small 1/0 requests (“lazy write”, “read ahead”).

[0143] The Aggregated File System 704 (AFS) is a file system driver. Itimplements the “Switched File System” aggregation mechanisms. Itpresents an array of file servers as a single file system bydistributing the metafiles and the data files stored among the fileservers. It also performs the function of aggregating data files andload balancing accesses between clients and the array of file servers.AFS further provides advanced NTFS-style features including Unicodenames, extended attributes and security descriptors, even if the filesystems that it aggregates do not have this support.

[0144] The Parallel Redirector 706 (RDR) is a file system driver. It issimilar to the Windows Workstation service, which exposes a file I/0interface and converts it to network file I/0 requests sent to a remoteserver. It uses multiple concurrent connections to the same networkserver in order to alleviate the inability of some CIFS implementationsto handle multiple pending client read and write requests on the samenetwork connection. In addition, the RDR is used to access thevirtualized “legacy” servers and to perform operations on aggregateddata files of the file system.

[0145] The data plane also includes a front-side network interface 710and a back-side network interface 712. A front-side and a back-sideTCP/IP protocol stack reside within the TCP/IP transport 708.

[0146] Various other services, such as DHCP, DNS, load-balancing,command-line and/or web-based management, SNMP, etc., may be included inor added to the architecture described above.

[0147] The implementation of the architecture described above can bearranged in many possible ways. For example, the network interfaces maybe implemented in hardware, while the rest of the data plane and the tworemaining planes are fully implemented in software. Alternatively,additional portions of the data plane may be implemented in hardware(e.g., by using Field-Programmable Gate Arrays, Application-SpecificIntegrated Circuits, switch fabrics, network processors, etc.), whilethe control plane 614 may be implemented in software. In addition, thecontrol plane 614 may be further implemented or accelerated in hardware.Moreover, it may be advantageous to implement portions of a certainplane (e.g., the data plane or the control plane) by providingaccelerated functions in hardware while maintaining the rest of theplane's functionality (such as setup, initialization and other slowfunctions) in software. In other embodiment, the Aggregated File System704 is provided, but the Virtual File System 702 is not provided. In yetanother embodiment one or more of the modules of the file switch areimplemented on the file servers of a NAS array.

[0148] One skilled in the art will easily recognize that various otherarchitectures for implementing a file switch are possible. In addition,while most of the particular choices made in implementing the fileswitch (such as those described above) are preferably driven by theperformance and cost targets of the file switch, all variousimplementations fall within the spirit of the present invention.

[0149] Operation of the Data Plane

[0150] In normal operation, the components in the data plane interactwith each other and with the Ethernet interfaces of the File Switch. Thefollowing steps illustrate the interactions between the components foran exemplary client session.

Exemplary Client Session

[0151] 1. Client connects to the file switch via the network interface710.

[0152] The TCP connection request is forwarded to SRV 718 via the TCP/IPtransport.

[0153] 2. Client logs in and attaches to a shared mount point exposed bythe switch.

[0154] The client's request arrives as a series of CIFS commands. SRV718 performs authentication of these requests without involving anyother data plane components.

[0155] 3. Client opens a file.

[0156] As the shared mount point exposed by SRV 718 is associated withthe file system owned by VFS 702, SRV 718 translates the request to afile system operation on VFS 702.

[0157] Next, VFS 702 consults a virtualization table stored in theconfiguration database and fmds the translated path for the file. Thispath may point to a file on a “legacy” file system handled by RDR 706 orto a file on an aggregated file system handled by AFS 704.

[0158] Next, VFS 702 retrieves the security descriptor for the file andperforms a security check to verify the client's right to open the file.If the check passes, the open request is forwarded to AFS 704 or RDR 706using the translated file path. Upon successful completion of the“open”, VFS 702 will request an opportunistic lock (op-lock) on the filein order to enable local caching of the file.

[0159] If the file is on a “legacy” file system, RDR 706 completes theopen operation through its CIFS connection to the NAS sever.

[0160] If the file is on an aggregated file system, the “open” requestis handled by AFS 704. Then, AFS 704 begins processing of the “open”request by issuing an “open” request to all mirror copies of themetadata file that represents the client's aggregated data files throughRDR 706. If at least one mirror copy is opened successfully, AFS 704completes the client's open request and starts calling RDR 706 to openthe data files that hold the client's data.

[0161] For each of the data files, RDR 706 picks one of its “trunked”connections to the corresponding NAS server to use for that file andsends a CIFS open request to that connection. Following an analogy fromthe telecom world, the use of multiple connections to the same target inorder to increase throughput is referred to in this specification as a“trunked” connection.

[0162] 4. Client reads metadata (e.g., directory information).

[0163] A client request to read file attributes, file size and similarrequests not related to data read/write are forwarded to SRV 718 and areconverted to file system operations on the metadata file correspondingto the specified user file. All of these requests go through the samepath as follows:

[0164] the VFS 702 forwards the requests directly to the same filesystem on which the file was originally opened.

[0165] if file is found on the AFS 704, the AFS 704 forwards therequests to RDR 706 as an operation on one of the mirror copies of themetadata file or to all mirror copies, if the operation involves amodification of the metadata file.

[0166] the RDR 706 converts the requests to CIFS requests and sends themto the NAS server.

[0167] 5. Client requests a data operation.

[0168] Client's data requests are converted by SRV 718 into “read”,“write” and “lock control” file I/O requests sent to VFS 702. Dataoperations on aggregated files are forwarded from VFS 702 to AFS 704.AFS 704 consults its aggregation table, compiled from data in theconfiguration database, computes how to distribute the requests amongthe data files that hold the client's data and forwards those requeststo the data files open on RDR 706.

[0169] 6. Client disconnects.

[0170] When the client disconnects, SRV 718 closes any files that wereleft open, thus providing proper closing of files on the servers, evenif the client does not close its file before disconnecting.

[0171] One skilled in the relevant art will easily recognize thatvarious modifications of this architecture can work well for theinventive file switch while preserving the spirit of the presentinvention. For example, more network interfaces 710 and 712 can beadded, and the two network interfaces can be replaced by a singlenetwork interface wherein the client traffic and the server traffic canbe separated by the TCP protocol stack. The TCP protocol stacks can bemerged together (in many conventional computer architectures there is asingle TCP/IP protocol stack that handles multiple network adapters) orseparated per network adapter.

[0172] In addition, multiple server-side SRV's 718 can be added in orderto process multiple network file protocols or different versionsthereof. Similarly, multiple client-side RDR's 706 can be added in orderto support multiple network protocols or multiple versions of suchnetwork protocol in interacting with the file servers.

[0173] Metadata File

[0174] A metadata file based switched file system aggregates filesacross multiple file servers of a NAS array in order to increaseperformance and to aggregate storage capacity. The subset of fileservers of a NAS array that are used to represent a single user file isknown as a file array. Every file contained in the aggregated filesystem has a corresponding file array.

[0175] The model of metadata file aggregation is based on the filearray. From the point of view of the client, an aggregated file is seenas a single file. However, the switched file system views the file asmultiple metafiles and data files stored on multiple file servers in thefile array. “Metafile based aggregation” refers to aggregating themetafiles and data files that together store the metadata and data fileof a specified user file.

[0176] There are two classes of properties of an aggregated file: stateand metadata. The state properties are managed internally by the fileswitch in memory. These properties are used to describe the currentstate of a file such as current oplock level, access mode, and cachemode. The metadata in general is shared between all clients of a singlefile. Each property has an associated aggregation class. The aggregationclass describes how a specific property is aggregated in relation to theelements of a file array.

[0177] Primary and Secondary Metadata File

[0178] The switched file system metadata for each aggregated file (alsocalled the user file) consists of two separate metadata files: a primarymetadata file and a secondary metadata file. The Primary metadata filecontains various properties about a specific aggregated file, such asthe aggregation parameters, file paths to the data files that store thecontents of the aggregated file, and file attributes. The metadata fileattributes represent the aggregated file attributes (file attributes,creation date and time, etc.). The primary metadata filename is the sameas the aggregated filename except it is prefixed with the letter ‘P’.

[0179] The secondary metadata file is used only (or primarily) to storethe aggregated size of the file. The size of the file is encoded in thefile's date/time attribute fields, which are retrieved through a fileget information request. The secondary metadata file contains no data.The secondary metadata filename is the same as the aggregated filenameexcept it is prefixed with the letter ‘S’. For file systems that do notsupport date/time attribute fields large enough to store the file size,the file size may be stored in the primary or secondary file's data.

[0180] In an alternative embodiment, only the primary metadata file iscreated and there is no secondary metadata file. In this alternativeimplementation, the aggregated file size is encoded directly in one ofthe primary metadata file's date/time attributes fields (e.g., thecreation date/time field).

[0181]FIG. 8 illustrates the contents of the primary metadata file 800in a preferred embodiment. At a minimum, the primary metadata file 800contains the following elements:

[0182] A header 801 field for storing genuine file attributes that areexposed to the user, such as creation, last access, and last writtendates and times. The header 801 is optional since much or all of theheader information may be stored in the directory entry for themetafile.

[0183] A metadata offsets field 802 for pointing to various portions ofthe metadata contained in the metadata file. This is used by theaggregated file system for quickly accessing the portions of themetadata. In alternate embodiments, the offsets field 802 can beeliminated if fixed sized fields or fixed position fields are used inthe metadata file.

[0184] An aggregation descriptor field 803 that contains a header of thedescriptor 804, a stripe-mirror map 811, and a data stream descriptor813. The header of the descriptor 804 further contains a flag thatindicates whether the metafile is valid. If the metafile is not valid,it should be ignored or updated to become valid.

[0185] A number of stripes field 805 for indicating the number ofstripes into which the corresponding user file has been divided.

[0186] A strip size field 806 for indicating the size (in number ofbytes) of each strip.

[0187] A number of mirror field 808, which indicates the number ofcopies (also called mirrors) of each stripe that are stored in a filearray.

[0188] A spillover field 809 for indicating whether there is anyspillover of the user file.

[0189] A number of data streams field 810 for indicating the totalnumber of data streams for the user file.

[0190] A matrix 812 of pointers to entries 830 in the data streamdescriptor. The size of the matrix is determined by the number ofstripes 805 and the number of mirrors 808 of the user file. The matrix812 contains an array of pointers (e.g., indexes into the data streamdescriptor), one for each distinct stripe-mirror of the user file, toentries 830 in the data stream descriptor 813. For example, if a filehas ten stripes and two mirrors, there will be twenty distinctstripe-mirrors for that file. Each instance of a stripe is sometimescalled a stripe-mirror instance, to emphasize that the data filecontaining that stripe instance is for a particular mirror of thestripe. Each entry 830 in the data stream descriptor 813 includes, inturn, the name 818 of (or a pointer to, or an identifier of) a fileserver in which a stripe-mirror instance of the user file is stored. Ifthe stripe-mirror instance overflowed the file server, then the entry830 also identifies a spillover segment with a pointer (index to nextdata stream) 815 to a next entry 830 that describes the spilloversegment.

[0191] A total file length field 820 for indicating the total aggregatedsize of the user file. This field is optional, although frequentlyhelpful.

[0192] The entries 830 of the data stream descriptor array each includethe following fields:

[0193] A state of data stream field 814 for indicating whether thestripe-mirror instance identified by an entry 830 is valid (containingcorrect data), invalid (e.g., containing out of date data) or does notexist.

[0194] An index to next data stream field 815 for linking to the entry830 for a spillover segment. The index 815 is null when there is nospillover segment.

[0195] A starting offset 816 within the aggregated user file forindicating the starting location of the segment or segment portionrepresented by the entry 830. When the entry 830 represents astripe-mirror instance (i.e., a segment of the user file) without aspillover segment, then the starting and ending offsets 816, 817 aredetermined solely on the strip size and the stripe number of the striperepresented by the entry 830. When a stripe-mirror instance has one ormore spillover segments, the starting and ending offsets represent thestarting and ending positions of each of the segments that forms thestripe-mirror instance. In an alternate embodiment, when a stripe-mirrorhas not spilled over, the field 816 is set to 0 and the field 817 is setto a special value (e.g., −1) to indicate a maximum value, which allowsthe system to avoid modifying the metadata every time data is written tothe end of the file, and allows multiple openers of the file to workmore efficiently together.

[0196] An ending offset 817 within the aggregated user file forindicating the ending location of the segment represented by the entry830.

[0197] A server name field 818 for indicating the name (or some otheridentifier) of file server in the file array that stores the filesegment represented by the entry 830.

[0198] A global unique identifier field 819, containing a global uniqueidentifier (GUID) for the data stream of a stripe-mirror instancecorresponding to the entry 830. The GUID is used for determining thedirectory structure in which the file segment corresponding to the entry830 is stored within a file server in the file array. The GUID, in ASCIIrepresentation, is also used as the file name of the data file(s) thatstores the stripe-mirror instance.

[0199] The metafile described above can be extended according to theneeds of the switched file system. For example, in an alternativeembodiment, a deleted file path field is included in the metadata filefor indicating the location of a user file that has been deleted, butnot yet removed from the file server. Saving the state of the deletedfile path enables the trash bin functionality (which allows deletedfiles to be recovered). In addition, a security descriptor field may beincluded in the metafile for indicating the access permission of a userfile. Other types of metadata that are not described above may also beextended according to the needs of the particular file aggregation andthe particular file system. The layout, structure and usage of themetadata are entirely up to the particular implementation of theswitched file system.

Aggregation with Metadata File

[0200] One objective of the present invention is to aggregate filesystem services provided by conventional file servers and present themto network clients as a single, large, very high performance networkfile system, the availability of which is many times higher than theavailability of each individual file server.

[0201] To achieve this objective, the file switch preferably aggregatesall operations of one or more network file protocols in such a way thatclients connected to the switch will not be able to distinguish itsoperation from the operation of a single network file server. Thisrequires the switch to aggregate all entities exposed by a typicalnetwork file protocol, in particular, the file system namespace,directories, and files. Clients connected to the file switch cannotobserve metafiles and data files separately. Rather, clients interactwith files, the files having both data (an array of bytes) and metadata(date, size, attributes, security descriptor, etc).

[0202] Rule-based Aggregation

[0203] The mechanisms that the file switch uses to achieve file systemaggregation are preferably implemented such that they can be driven froma set of rules and policies defined on the file switch.

[0204] There are several attributes that make rule-based aggregationdesirable. First, it allows a storage administrator to specify differentways of aggregation for different sets and/or types of files, therebyeasily tuning the characteristics of the system to the intended use andthe specific access patterns for different data. Second, it allows thefile switch to operate with more deterministic timing by eliminating theneed to consult external devices during normal operation.

[0205] In addition, rule-based operation allows multiple file switchesto aggregate and be put in front of the same set of servers without thefile switches having to interact with each other, except to synchronizethe set of rules and policies whenever they are changed. This loosecoupling between file switches that aggregate the same set of fileservers makes it possible to scale access bandwidth by orders ofmagnitude, simply by adding file switches whenever needed.

[0206] Finally, since file switches are in an excellent position totrack usage patterns internally, they can be configured to adjust theaggregation rules (discussed below) automatically in accordance withpolicies specified by the system administrator and observed usagepatterns. As a result, the file switches can optimize in wide marginsthe distribution of files and data among the file servers to achievesmooth and adaptive behavior of the network storage system as a whole.

[0207] Namespace Aggregation

[0208] Namespace Rules

[0209] In order for a file aggregator to redirect a file operation tothe appropriate NAS array, it uses a set of namespace rules (also calledthe namespace aggregation rules) to generate the corresponding NAS arrayfile path. Using the given file path accessed by a client and matchingnamespace rule, the NAS array file path can be generated by using a pathreplacement process. Before using path replacement, the aggregator mustselect the matching namespace rule for the given file path. Once therule is selected, the aggregator uses a path replacement process togenerate the proper NAS array file path. The path replacement processreplaces the client's file path with the NAS array file path.

[0210]FIG. 9 illustrates a rule-based namespace aggregation by theinventive file switch to aggregate multiple file servers under a commonfile system namespace. The rules for namespace aggregation arepreferably defined as a table of path correspondences. The first columnspecifies the names visible to the clients, the second column specifiesthe name of the file server and, optionally a shared mount point on thatserver, in which the files actually reside. A file switch is shownconnected to three file servers 908, 909 and 910. Loaded within (orotherwise accessible by) the file switch is a rule table 904 thatspecifies three rules 905, 906 and 907. The path names 901, 902 and 903of incoming file requests, such as file open requests, initiated by anetwork client are compared to the name-mapping rules in the firstcolumn (preferably the comparison is done either by matching longestprefixes first, or by applying the rules in a predefined order ofpriority, so that overlapping pathnames can be specified). If a match isfound, the matching portion of the file base path is replaced with thename from the second column and the request is forwarded to the new pathfor processing. Once a file is open on the target server, all furthertransactions related to this file are switched to that server.

[0211] For example, rule 905 specifies that the \ENG subtree of thecommon namespace is to be mapped to the server 908. File 901 will matchthis rule and therefore will be switched to the server 908 where it willarrive with a modified path. However, rule 906 specifies that a subtreewithin the \ENG subtree, namely \ENG\SW, is to be mapped to a differentserver, server 909. File 902 satisfies this rule and will therefore beswitched to server 909, where it will arrive with a modified path. Inaddition, rule 907 specifies that the \ACCT subtree is to be mapped toserver 910. This rule will drive the switching of file 903 even thoughthis file resides in a subdirectory of the \ACCT subtree (because of theprefix match).

[0212] In addition to base path, other namespace mapping rules arecontemplated. For example, a rule may specify that all files with agiven extension (and, optionally also under a given subtree) areswitched to a specific server. For example, a rule(*.mpeg-->\\srv3\dir6) will cause all MPEG files to be sent to thesubdirectory dir6 on server SRV3 910, no matter where in the logicalnamespace these files reside.

[0213] One skilled in the art will recognize that although the aboveexample illustrates a method for mapping a particular file type to aparticular directory of a specific server, this method can begeneralized to include a “file system name” plus a “target directory”.For example, the file system name may identify a NAS array 310 or alegacy server 313. This generalized method is used in the determinationof NAS array as described below in FIG. 14.

[0214] It should be noted that the new path created by the applicationof the namespace aggregation rules is the file path for the metadatafile corresponding to the specified user file. Access to data within theuser file is redirected to other file servers, and to specificdirectories and data files within those directories, in accordance withthe metadata in the metadata file. This will be explained in more detailbelow.

[0215] Note that by aggregating the namespace of multiple file serversinto a common namespace, the file switch achieves a function similar towhat available distributed file systems do without requiring anyproprietary client-side software.

[0216] Name of a Data Stream

[0217] Each aggregated file consists of one or more data streams thatcontain the file's data. The number of data streams depends upon thenumber of stripes and mirrors for the specific data file, as well as thenumber of spillover fragments (as explained in more detail below). Thename of a data stream is the ASCII code (i.e., the ASCII representation)of the global unique identifier (GUID) stored in the corresponding entry830 of each data stream. This ensures the name for each data stream isunique because of the uniqueness of the GUID.

[0218]FIG. 15 illustrates an example where the above naming methodologyis observed. The data stream names for the document myFile.doc 1500 areformed using the ASCII code of the GUID of the corresponding datastream. For example, the name for the first data stream on file server1501 is the ASCII code of the GUID for this entry (namely GUID_ASCII_1)and similarly, the names for the first data stream on file servers 1502to 1506 are the ASCII codes of the GUID for the respective entries,namely GUID_ASCII_2, GUID_ASCII_3, GUID_ASCII_4, GUID_ASCII_5 andGUID_ASCII_6. Note that the mapping is configured and performed on thefile switch. The clients don't need to know, and in fact have no way ofknowing, the mapping and do not need to be reconfigured if the mappingis changed.

[0219] Data Aggregation Rules

[0220] The ability to aggregate data files among multiple servers and todo so safely in a concurrent environment enables the file switch todistribute the data of the aggregated file on multiple servers, therebyachieving both parallel operation and high availability. The sameprocess can be viewed as the file switch aggregating the contents of themember files into a single file that it presents to its network clients.

[0221] Most network file protocols represent data file as contiguousarrays of bytes. This means that the techniques required to distributethe data for each individual file are not different from the techniquesrequired to distribute the data for an array of hard disks. Inaccordance with the present invention, the methods for doing so,including striping, mirroring and other variations of RAID, are appliedto distributing data of individual files across a set of file servers.

[0222] FIGS. 10-12, described hereinafter, respectively illustratemirroring, striping, and spillover as implemented by the presentinvention. As these mechanisms exist conventionally, a representation ofthe clients and servers is not believed necessary. It is noted, however,that these mechanisms are performed by the present invention based onswitching file protocol transactions that take place in the file switch(represented by the arrow in each of these figures), rather than APIfunctions that take place on a local machine, typically the client.

[0223] Mirroring

[0224]FIG. 10 illustrates data aggregation through mirroring in aswitched file system. In this example, the file switch (not shown)aggregates member files 1001, 1002, 1003 and 1004, all preferablyresiding on different file servers, into a single aggregated file 1000,presented to the clients. The member files 1001 through 1004 containidentical data, which the switch presents as contents of the aggregatedfile 1000.

[0225] When the client initiates a file open transaction, the switchaggregates that transaction (as shown in FIG. 10) and opens either oneor all of the member files 1001 through 1004, depending on the type ofoperation that is to be performed subsequent to the file open. When theclient initiates a file open and a file read transaction, the fileswitch selects, preferably randomly, one of the file servers on whichthe member files reside and switches the open and read transactions toit. That server executes the open and read transactions and returns theresponse to the switch; the switch forwards the response to the client,thus completing the read transaction requested by the client. With thismechanism, if multiple clients try to read the same file 1000, the fileswitch will direct their transactions to different member servers atrandom (or in accordance with predefined criteria, such as loadbalancing criteria). The switch thus balances the load among these fileservers. In addition, the clients can experience up to four timesincrease in performance compared to a situation where the file 1000 isstored on a single server.

[0226] When a client initiates a file write transaction, the switchaggregates the transaction by replicating the user data into all of themember transactions. As a result, all member files 1001 through 1004 areupdated synchronously with the same data. Since all member transactionsexecute in parallel, this does not significantly degrade the performanceof write transaction on the aggregated file compared to writetransactions on a file stored on a single server.

[0227] Finally, when a client initiates a close transaction, the switchaggregates it in a manner similar to the open transaction and closes allmember files.

[0228] One other significant advantage of file mirroring is that theabove transactions can be completed successfully even if one or more ofthe member file servers become unavailable. Open, write and closetransactions are switched to all available servers; read transactionsare switched to any one of the available servers. This way, as long asat least one of the member files is online, the file system as a wholeand the aggregated file 1000 in particular remain available to allclients.

[0229] Striping

[0230]FIG. 11 illustrates data aggregation in a switched file systemthrough striping by a file switch. In this example, a user file 1100contains 6 file strips 1105 through 1110. The file switch (not shown)distributes the user file into 4 stripes 1101 through 1104, allpreferably residing on different file servers, according to apredetermined number of stripes 805. The stripes 1101 through 1104 inthis case contain different, non-overlapping strips 1105 through 1110,which the file switch presents as a contiguous aggregated user file1100.

[0231] When a file switch receives a file open transaction from aclient, it aggregates that transaction (as shown in FIG. 11) and opensthe corresponding metadata file. From the metadata file, the file switchdetermines the number of stripes and the file server locations of thedata files containing the stripes. By placing an appropriate lock on themetadata file, the file switch can furthermore prevent other clientrequests from interfering with the operation of the current clientrequest.

[0232] When the client initiates a file read transaction, the switchaggregates this transaction by executing the following steps. First,determining based on the strip size and the requested starting offsetand the requested transaction size, which of the member servers will beinvolved in the transaction, and at what starting offset and what amountof data each of them must read. The switch then issues the membertransactions to the selected servers and aggregates the results byensuring that data arrives at the client in the right reconstructedorder. The client receives the aggregated header for the response,followed by all of the data requested, in the correct order.

[0233] One skilled in the art will recognize that the write transactionin this case is executed in a manner similar to the read transactiondescribed above, except that the data is distributed as illustrated inFIG. 11, instead of being assembled as was the case with the readtransaction. Finally, when a client initiates a close transaction, theswitch aggregates it in a manner similar to the open transaction andcloses the corresponding metadata file, as well as any of the stripedata files that have been opened.

[0234] In the case of data aggregation through striping, both read andwrite transactions are aggregated by submitting corresponding read andwrite transactions for smaller amounts of data to multiple memberservers in parallel. This results in a respective increase ofperformance, which the file switch can deliver to each individualclient, as well as to an excellent load balancing in the case ofmultiple clients accessing the same file. In addition, as multiplestudies have shown, striping tends to resolve the problem ofhotspotting.

[0235] Spillover

[0236]FIG. 12 illustrates data aggregation through spillover. Thespillover mechanism is preferably used to aggregate storage capacity,preferably in conjunction with one or more of the other mechanismsdescribed herein. The spillover is especially useful in cases where oneor more of the member servers for an aggregated file unexpectedly runout of disk space while the file is open. The figure illustrates anaggregated file 1200, comprising two member files 1201 and 1202,preferably residing on different file servers. As seen from the figure,sections 1, 2, 3, and 4 of the aggregated file 1200 reside in memberfile 1201, while the remaining sections 5 and 6 reside in member file1202.

[0237] Spillover happens when the file switch, in the process of writingdata into a file unexpectedly discovers that the target file server isabout to run or has run out of disk space. In such case, rather thanfailing the write transaction, the switch may elect to open a new memberfile on another server and continue writing into it. The contents of thetwo files are concatenated to present a common contiguous byte array inan obvious way. One skilled in the art will recognize that the spillovermechanism can be applied to the second file as well, creating anarbitrarily long chain of member files, so that all disk capacity in thesystem can be fully utilized if needed.

[0238] The file switch switches file transactions to spilled-over filesas follows. For read and write transactions, the file switch looks atthe starting offset and the length of the payload to be read/written andswitches the transactions as follows:

[0239] (a) if the payload fits completely within the first member file(e.g., segments 1 and 2 from file 1200), the file switch switches thetransaction to the first server.

[0240] (b) if the payload fits completely within one of the spillover(second and further) member files (e.g., segment 5 from file 1200, whichis stored in the beginning of the member file 1202), the file switchswitches the transaction to the server on which that member fileresides. The switch also modifies the parameters of the transaction bysubtracting from the starting offset for the transaction the startingoffset of the member file within the aggregated file. In our example,segment 5 is at offset 0 in file 1202, so four segments should besubtracted from the request, resulting in a request to read the firstsegment from file 1202.

[0241] (c) if the payload spans multiple member files (e.g., segments 4and 5 from file 1200), the file switch replicates the transaction to allservers on which portions of the request reside, modifying the startingoffset and length of each transaction. Upon receiving the responses, thefile switch reconstructs the data in the correct order (similar to theway this is done for striping) and sends it back to the client.

[0242] In order for the spillover mechanism to function, the metadatafile stores the range of data file and the location of the member filesin the file system (i.e., the server on which each member file residesand the file name and file path of the member file). This sameinformation is obtained from the metadata file during read and write orupdate operations.

[0243] According to the present invention, the file switch aggregatesdata file on a file-per-file basis. In this way, different files can beaggregated in different ways using different combinations of striping,mirroring and other data aggregation techniques to achieve optimalbalance between performance, storage utilization and the desired levelof data availability.

[0244] It is well known that the effectiveness of striping, mirroringand other data aggregation techniques when applied to block devices,such as in RAID or parallel file systems, can be greatly diminished bythe fact that no single solution can fit all types of files and accesspatterns. By way of example, streaming video can be striped veryeffectively over a large number of devices, since streaming data isusually being read in large segments. On the opposite side of thespectrum, HTML files are typically only a few kilobytes large and not agood target for striping. Therefore, the present invention utilizesaggregation rules (also called the data aggregation rules) to configurethe file switch with different data aggregation parameters for differenttypes and/or sets of files.

[0245] Syntax of Data Aggregation Rules

[0246]FIG. 13 illustrates the syntax of data aggregation rules andprovides examples of such rules. The preferred syntax 1300 defines a setof aggregation parameters, namely, number of mirrors, number of stripes(i.e., the preferred number of file servers across which the stripes arestored) and strip size, which are selected for a given set of filesbased on each file's path (location in the aggregated namespace) andtype (recognized by the file extension/suffix).

[0247] Rule 1301 shows typical parameters for MPEG files locatedanywhere in the file system. The rule is selected for any file path, butonly for files whose filename extension is MPEG, and it definesmirroring by 2, striping by 32 and a strip size of 16 KB. With thisrule, any MPEG file will be mirrored once (two copies of the data willexist in the system) and striped across 32 file servers, with a filestrip size of 16 kilobytes.

[0248] Rule 1302 shows typical parameters for HTML files locatedanywhere in the file system. The rule is selected for any file path andonly for files whose filename extension is HTML, and it definesmirroring by 64 and no striping. With this rule, any HTML file will bemirrored on 64 file servers, which allows load balancing when read bylarge number of clients simultaneously (which is the typical accesspattern for HTML files on a HTTP server).

[0249] Rule 1303 shows typical parameters for Microsoft Word documentfiles located anywhere in the file system. The rule is selected for anyfile path and only for files whose filename extension is DOC, and itdefines mirroring by 3, striping by 8 and a strip size of 8 KB. Withthis rule, any document file will be mirrored twice (three copies of thedata will exist in the system for higher availability) and stripedacross 8 file servers, with a file strip size of 8 kilobytes. Since mostsuch documents typically have file sizes between 32 KB and 100 KB, thisrule provides moderate (e.g., 4×) improvement in performance for eachindividual client, and lowers the probability of hotspottingsignificantly since each file is spread across a total of 24 fileservers (if that many file servers are available) without wasting toomuch storage space.

[0250] Rule 1304 shows a desired set of aggregation parameters forsoftware source code files that contain valuable intellectual propertywhile being each small in size. The rule applies to any file in the\CODEBASE subtree of the aggregated namespace, and defines mirroring by4 and no striping. This provides moderate performance increase (e.g.,4×) during program compilation and build, which is the usage patternwhere hundreds of files are being read in a batch process and providesexcellent protection from data loss due to server failure.

[0251] Finally, rule 1305 is a modification of rule 1304 that optimizesthe use of storage space in the \CODEBASE subtree. This rule recognizesthe fact that source code directories often contain intermediate objectcode files (with file extension of OBJ) which are a byproduct of thecompilation process and can easily be reconstructed if lost. The ruledefines an exception from rule 1304, namely that any file in the\CODEBASE subtree that has a filename extension of OBJ will be neithermirrored nor striped. When used together, rules 1304 and 1305 can easilyprovide optimal storage characteristics for a software engineeringdepartment.

[0252] In another embodiment, the data aggregation rules containadditional parameters. In particular, the syntax of the data aggregationrules in this embodiment is:

[0253] (Path, Type)→(N Mirrors, N Stripes, Strip Size, operationalparameters, caching parameters)

[0254] The operational parameters may include, for example, a lockredundancy parameter that specifies the number of file servers on whichfile lock are to be replicated. The caching parameters may include a“read ahead enabled” parameter, which indicates whether read aheadcaching (i.e., retrieving and caching data from a file before it hasbeen requested by an application running on the client computer) isenabled for the files to which the aggregation rule applies. The cachingparameters may include a “write behind/write through” parameter, whichindicates (for the files to which the rule is applicable) whether newand updated data is lazily written back to the file servers, or iswritten back immediately. The caching parameters may also includecaching parameters that specify one or more of a maximum cache size, amaximum caching time, a maximum amount of dirty data that can be cachedwithout writeback to the file server (if write behind is enabled), andso on.

[0255] Summary of Data Aggregation Rules

[0256] This section has described the various mechanisms, algorithms andother elements of the present invention used to achieve the desiredbehavior of the file switch, namely the ability to aggregate multipleindependent file servers into a single, highly scalable switched filesystem.

[0257] One skilled in the art will easily recognize that the mechanismsdescribed in this section can be beneficially applied simultaneously tothe same file. For example, mirroring and striping can be combined toincrease both performance and availability of a single file; further,spillover can be added to the same file in case some of the file serversrun out of storage space. Moreover, one skilled in the art willrecognize that other data aggregation techniques, for example RAID4 andRAID5, can be implemented in a file switch in addition to or instead ofthe mechanisms described herein.

[0258] Directory Aggregation

[0259] Namespace aggregation as described above is an easy way todistribute files among different servers, and also to add a new serverto an existing system. However, this technique alone may not besufficient to aggregate seamlessly the storage capacity of multiple fileservers. For example, with namespace aggregation alone it may not bepossible to tell how much free disk-space is available on the aggregatedfile system.

[0260] Since different directories are mapped to different servers, afile that cannot be stored under the \ENG subtree for lack of room maybe successfully stored under the \ENG\SW subtree, which resides on adifferent server. Thus, even when the system as a whole has plenty ofavailable storage space, particular file operations in particular placesin the namespace may not be able to execute successfully withoutextensive human intervention.

[0261] Directory aggregation resolves the above issues by making itpossible to distribute files that reside in the same aggregateddirectory among different servers. This ensures that the files from alldirectories of the combined namespace can share all of the availablefree disk space.

[0262] Directory Structure of Metadata File

[0263] In order to determine the directory structure for storing themetafile of a user file in the group of file servers, the file switchneeds to construct the following:

[0264] [NAS array][file server][directory path][filename]

[0265]FIG. 14 illustrates a method for constructing the directorystructure of a metafile. The method consists of the following key steps:

[0266] 1. Determine NAS array 1404: The aggregator needs to firstdetermine which NAS array should be used. This determination is based onthe namespace rules. The file path being accessed is mapped to aspecific NAS array and directory path in accordance with the namespacerules, as described above.

[0267] 2. Determine File Servers 1406: The file server (morespecifically the set of file servers) that contains the metadata file isdetermined by using a hash function (e.g., by applying it to the userfile name) to identify a first file server. The set of additional fileservers for storing redundant copies of the metadata file is determinedsimply by selecting the “next” N−1 file servers in the NAS array, when atotal of N file servers are needed. If the number of servers in thearray is reached, the counting wraps around to the first server. Themetadata redundancy N is independent of the number of stripes and numberof mirrors. N can be set as a constant on the NAS array, or be set persubtree in the namespace aggregation rules (e.g., by adding ametadata_redundancy field to each aggregation rule.

[0268] 3. Determine Directory Path 1408: The directory path for ametafile is calculated using the namespace aggregation rules and thefile path (of the associated user file) provided with the request. Partsof the file path may need to be replaced depending on the namespaceaggregation rules. The constructed directory path is replicatedaccording to the number of redundant metafiles, which in someembodiments is defined by the namespace aggregation rules.

[0269] 4. Determine Metafile Names 1410: The file names of the primaryand secondary metafile stored on the file server are the same as theuser file name, with a prefix of “P” for the primary metafile and aprefix of “S” for the secondary metafile respectively.

[0270] The NAS array in which the metafile is to be stored is identifiedby the namespace aggregation rules. There may be multiple NAS arrays ina given file system. Each NAS array is responsible for specificdirectories and files as described by the rules. Directory aggregationapplies to a specific NAS array; not all NAS arrays as a whole. The fullset of file servers that makes up the NAS array must be known to thefile switch (e.g., a background service may keep track of thisinformation). Each file server in the array is identified by itscomputer name on the network and a share (server's file system mountpoint) in which files should be stored. In order to access a file storedon a given server, the pair <server, share> is needed. Since everyserver preferably participates with a single share in the NAS array, thepair <NAS array, server> is sufficient to identify <server, share>. The<server, share> pair for each server that participates in the NAS arrayis listed in the configuration of the NAS array.

[0271] A hash function is used to determine the first file server in theNAS array that contains a specific metafile. The hash function isapplied to the name of the file (preferably not including the filepath). The value of the hash is used to determine which file servercontains the first occurrence of the metafile. The hash function isconfigured to produce a value in a range of numbers equal to the numberof file servers in the NAS array, where each value in this rangerepresents one of the file servers in the array. This mechanism evenlydistributes metafiles across the NAS array. The hash function, used inconjunction with the namespace aggregation rules, determines the exactsubset of file servers containing the specific metafile.

[0272] To one skilled in the art, it will be apparent that it ispossible to set the metadata redundancy N to 1, in which case thedirectory aggregation achieves only distribution without redundancy. Itis also possible to set the hash function to always return the valueassociated with the first server in the NAS array, thereby achievingonly metadata redundancy. However, it is highly beneficial to use bothredundancy and distribution of the metafile, to improve data securityand availability.

[0273] The components of the array configuration are used to aggregatefiles and directories across the NAS array. Below is a simple example:

[0274] Namespace rule: \ENG\DOCS\*.*>NAS3\DIR4\DATA

[0275] Client requested file: \ENG\DOCS\JOHN\myFile.doc

[0276] Value of file hash: second server in NAS array

[0277] Configuration entry for the second server in the NAS array NAS3:server SRV2, share SH1

[0278] Using the above information, the client file path“\ENG\DOCS\myFile.doc” is translatedinto“\\SRV2\SH1\DIR4\DATA\JOHN\myFile.doc”on the third NAS array, NAS3(“SRV2” is the name of the second server in the NAS array NAS3). Thedirectory “\DIR4\DATA\JOHN”, if it doesn't already exist, is created onall members of the NAS array, not just the members containing themetafile for file “myFile.doc”, to support directory enumeration andmetafile redundancy.

[0279] This example pertains to both opening and creating files. Whenaccessing the file on the NAS array, the metadata files involved are:

[0280] NAS3\DIR4\DATA\JOHN\PmyFile.doc—primary metadata file

[0281] NAS3\DIR4\DATA\JOHN\SmyFile.doc—secondary metadata file

[0282]FIG. 15 illustrates a graphical representation of the storage ofthe user file and metadata files for an aggregated user file named“myFile.doc”. There are six file servers 1501 to 1506 in the file array.The user file is divided into six stripes. In this example, no mirrorsof the user file are shown.

[0283] The primary and secondary metadata files (PmyFile.doc andSmyFile.doc) are stored in the first file server 1501 of the array. Themetadata files are replicated one time in file server 1502 to provideredundancy.

[0284] Directory Structure of a Data File

[0285] The data files are preferably stored on servers of the same NASarray as the metadata files but in a different directory sub-tree,separate from the metafiles. A “file array” is the subset of fileservers in a single NAS array that stores the contents of a specificfile. This section describes how to create the directory structure ofdata files within a file array.

[0286] User File Data Distribution Mechanism

[0287]FIG. 16 illustrates a preferred embodiment for distributing thedata of user files in a NAS array. The method consists of the followingsteps:

[0288] 1. Determine NAS array 1604;

[0289] 2. Determine the number of file servers 1606;

[0290] 3. Select File Servers and Perform Load Balancing 1608;

[0291] 4. Determine and Handle Spillovers 1610;

[0292] 5. Create Global Unique Identifiers (GUID) 1612;

[0293] 6. Determine File Path with GUID 1614;

[0294] 7. Create Data Stream Filename 1616;

[0295] The method starts in block 1602 and moves to block 1604 wheremapping of a user file to the proper NAS array is performed. The methoduses the namespace rules described above. This method is the same as fordetermining the NAS array for storing the metafile.

[0296] In block 1606, the number of file servers to be used to store thedata of the user file is determined. The method applies the aggregationrules, which specify the number of stripes and the number of mirrors. Inone approach, the number of servers is computed by multiplying thenumber of stripes by the number of mirrors. However, in the event thatthere is an insufficient number of file servers to store each mirroredstripe of the user file, multiple stripes (i.e., stripe instances) canbe stored in a single file server.

[0297] In block 1608, the number of file servers computed in block 1606is selected from the NAS array. There are numerous selection methodsthat can be applied to select the file servers for achieving the goal ofload balancing in storing the user file. In one selection method, calledthe round robin method, each file server within the NAS array isselected sequentially for storing a mirror-stripe file for the userfile. When multiple copies of each stripe are to be stored, eachinstance or copy of the stripe must be stored on a different fileserver. In other selection method, based on the available disk space onthe file servers, the file server with the largest available disk spaceis selected first, and then the file server with the next largestavailable disk space is selected second. The process continues until allthe stripes of the user file are stored. Yet another method forselecting the file servers can be based on the historical loadstatistics of a particular file server. Yet another method for selectingthe file servers can be based on the response time of the file servers.

[0298] In block 1610, the method determines and handles any spilloverfragments of the stripes that form the user file. While aggregatingfiles to multiple devices, over time some of the device's storagecapacity may become exhausted. As a result, the file aggregation mayfail and cause disruptions in the systems network. To avoid suchfailures, file aggregation includes spillover. This is a mechanism thatallows the aggregator to use a different storage device (i.e., fileserver) when one or more of the devices run out of storage space. Eachfile server's storage capacity must be monitored using a specificthreshold. The threshold varies depending on the storage capacity of thefile server. The threshold is needed so a portion of the storage ispreferably reserved for file spillover information and metafiles. Notethat when the user file is first created, the determination step willshow that no spillover fragment exists, and hence the handling spilloverstep will not be performed.

[0299] When the file aggregator detects that a particular file serverhas reached its threshold (i.e., the file server's disks are full), adifferent file server is designated for all subsequent data belonging tothe accessed data file. One approach to storing the spillover fragmentsof a user file is to store the spillover data file in the subsequentfile server, in a predefined sequence of the file servers with the NASarray. The sequence of the file servers wraps around when the last fileserver is reached. Whenever possible, the aggregator preferably avoidsstoring a spillover fragment of a given stripe's mirror on the sameserver where another mirror (or fragment thereof) of the same stripe isalready stored; this allows the aggregator to preserve data redundancy.

[0300] By allowing directories and their contents to spillover on theservers, the capacity of the entire NAS array can be used for filestorage.

[0301] For a given file, there is either spillover or no spillover, asindicated by the spillover flag 809 in the metafile for the user file.If there is no spillover, the flag indicates that there is no spilloverand that each stripe-mirror instance is represented by a single datastream. If there is spillover, the flag indicates so, and the spillovercontents of a stripe are stored on another server using a new GUID (seeGUIDs below); the determination of the file path of the new data streamis described below, with reference to FIG. 17. As long as at least onestripe-mirror instance has been spilled over, the spillover flag is set(in some embodiments, however, the spillover flag may be eliminated).The primary metadata file is updated to include pointers to the full setof spillover fragments. In addition, all redundant metafiles must beupdated to include entries 830 for each spillover fragment.

[0302] To indicate that an aggregated file has spillover, its primarymetafile is updated with the following information:

[0303] Spillover flag 809 is set, to indicate that the file has at leastone spillover.

[0304] Total number of data streams, which is stored in field 810 of themetadata file, as shown in FIG. 8. This parameter indicates the totalnumber of data streams for the aggregated file, including the firstfragments of each stripe-mirror instance and any spillover fragments ofany stripe-mirror instance.

[0305] List of all data streams which include (<stripe #>, <mirror #>,<start offset>, <end offset>, <logical device name>). More particularly,each data stream is represented by an entry 830 of the data streamdescriptor 813 in the metadata file, as shown in FIG. 8. The entry 830for the initial fragment of a stripe-mirror instance is identified inthe stripe-mirror map 81 1, more specifically by a data stream indexvalue stored in the matrix 812 by stripe number and mirror number. Oncethe first data stream has been spilled over, the first spilloverfragment is linked to by the “index to next data stream” 815, and ifthere are any additional spillover fragments for the same stripe-mirrorinstance, these are found by following the links in the index field 815of successive entries 830.

[0306] The spillover information in the metafile is preferably stored inthe order that the spillovers occur. A full stripe of a file is aconcatenation of all of the stripe fragments, including an initialfragment file and zero of more spillover fragment files, in the orderthat they are listed in the metafile. Each fragment file is stored onone of the NAS devices, as indicated by the server name 818 in the entry830 representing the fragment file. The file name for the fragment isindicated by the GUID field 819 in the entry 830 for that fragment file.

[0307] When accessing a file contains spillover data, the file switchchecks if the needed data is on the regular file server for a particularstripe, or a spillover file server, or both. The file's metadata is usedto determine which file servers contain the spillover data. There may beany number of spillover file servers in the NAS array.

[0308] In block 1612, the global unique identifier (GUID), a value thatis 16 bytes long in a preferred embodiment, is created for each distinctfragment (data stream file) of the user file. The length of the GUID maybe different in other implementations. The GUID for each data streamfile is stored in a descriptor field 819 of the corresponding entry 830in the metadata file. FIG. 17 illustrates a method for creating theGUID. The inputs 1702 for creating the GUID consist of a unique MACaddress of a network interface, a time stamp and a sequence counternumber. In other embodiments, other information or additionalinformation (e.g., the filename) could be used as input to the GUIDfunction 1704. The MAC is the unique network address of one of the fileswitch's network interfaces (and uniquely identifies the file switchamong all other file switches); the time stamp indicates the time of theuser file creation and the sequence counter counts the number of filescreated by the file switch. The GUID function 1704 combines the inputsto create a unique bit stream that is written into the GUID 1706. TheGUID is preferably unique among all the GUIDs generated by any fileswitch, on any NAS array, on any server.

[0309] In block 1614, the file path, within a file server, for each datafile (i.e., each stripe-mirror instance file and spillover file) isdetermined using the GUID for that data file. FIG. 17 illustrates oneapproach to implement this step. In block 1706, the GUID is divided intomultiple bitfield segments, herein called indexes, namely index 1, index2 and up to index n. The directory path to the data stream file isformed by concatenating a subset of the indices to form a file path,with each utilized index comprising a directory name in the file path.For example, the GUID of a data file may contain indices A, B, C, D andE, as well as other portions not used in the file path. In oneembodiment, each index from the GUID comprises one or two ASCIIcharacters. The file path for the data file is then \A\B\C\D\E\filename.As shown in 1708, each index from the GUID forms the name of a directoryin the file path of the data stream. By forming the file path of each ofthe data streams in this way, the data streams are automatically andrandomly (or pseudo-randomly) spread over a large number of distinctdirectories, thereby preventing large numbers of data streams from beingstored in a single directory. Having large numbers of data streams inthe same directory could have an adverse impact on system performance,and this file path forming mechanism avoids that potential problem.

[0310] In block 1616, the file names of all the data streams of eachstripe-mirror instance of the user file in the file array aredetermined. In normal operations, each aggregated file consists of oneor more stripe-mirror instances. The number of stripe-mirror instancesdepends on the number of stripes and mirrors for the specific user file.The number of data streams for each stripe-mirror instance depends onthe number of spillovers for the specific stripe-mirror instance. Thedata streams are named using the ASCII code of the GUID associated witheach corresponding data stream, with two ASCII characters for each byteof the GUID. This was described above in detail with reference to FIG.15. Other methods can be used to convert the GUID into valid filenamesusing characters allowed by the file servers; one such method is toconvert the number in a base-62 system, where the digits are 0-9,followed by all uppercase letters of the English alphabet, followed byall lowercase letters (10+26+26).

[0311] Note that the mapping of the data files (that together form thedata for an aggregated file) to file servers, and to specificdirectories on the file servers is performed by the file switch. Theclients don't need to know, and in fact have no way of knowing, themapping and do not need to be reconfigured if the mapping is changed.

[0312] After the step of determining data stream file names in block1616, the method ends at block 1618.

[0313] It should be pointed out that in other embodiments, the steps ofFIG. 16 may be performed in a different order. Further, many of thesesteps may be performed or re-executed each time the user file increasesin size sufficiently to require that addition of a new data stream forthe user file.

EXAMPLE

[0314] The following example illustrates how the directory structure forthe user file is determined. This example assumes the following:

[0315] One file aggregator (i.e., file switch) and 2 different NASarrays NAS1 and NAS2. Each NAS array contains 8 file servers. The namesof the file servers in NAS1 are NAS1_SRV1, NAS1_SRV2, NAS1_SRV3, etc.The names of the file servers in NAS2 are NAS2_SRV1, NAS2_SRV2,NAS2_SRV3, etc.

[0316] The following namespace rules are defined: Rule 1:\ZF\ENG\DOC\*.* → NAS1\DOC_DIR Rule 2: \ZF\ENG\DESIGN\*.* →NAS2\DESIGN_DIR Rule 3: \ZF\ENG\TRAINING\*.MPG → NAS2\MOVIES

[0317] The following aggregation rules are defined for NAS1: Rule 1:\DOC_DIR\*.* → { stripe (4, 8192), mirror (1) }

[0318] The following aggregation rules are defined for NAS2: Rule 1:\DESIGN_DIR\*.* → { stripe (4, 8192), mirror (1) } Rule 2: \MOVIES\*.* →{ stripe (8, 16384), mirror (0) }

[0319] Assuming the client requests to access the file“\ZF\ENG\DOC\GEARS.DOC”:

[0320] According to namespace rule 1, this path is mapped to the firstNAS array NAS1 to the directory “DOC_DIR”. The application of thisnamespace rule identifies the location where the metadata file for theuser file is located.

[0321] According to the aggregation rule 1 for NAS1, “DOC_DIR” isstriped over 4 servers, each stripe is 8 K and each stripe is mirrored 1time on the other 4 servers in the NAS array.

[0322] Let HashFunction(GEARS.DOC)=0. In this case, the first servercontaining the file “GEARS.DOC” is NAS1_SRV1. Additional file servers,for additional stripes and mirrors are identified using this firstserver as a starting point. Alternately, the file servers to be used tostore the data files are identified using a load balancing function.Further, each copy of a stripe data file must be stored on a differentfile server than the other copies of the same stripe, in order toprovide protection against file server failures, and to provide paralleldata paths for improved throughput.

[0323] A separate GUID is computed for each distinct data stream of auser file. Thus, a respective GUID is computed for each data stream of astripe-mirror instance, and if there are spillovers, a separate GUID iscomputed for each spillover segment. From the GUID for each data stream,a file path is generated, and each data stream is stored in thedetermined file server at the file path determined from its GUID.

[0324] Isomorphic Trees

[0325] In order to implement directory aggregation, described below, theaggregated directory structure is preferably present on all servers ofthe NAS array. Each file server preferably has the same directorystructure (also called a directory tree) under the share exposed forthis server in the NAS array. Having isomorphic directory trees enablesmetafiles to be stored on any server in the NAS array. Each file serverneed not have the same metafiles.

[0326] In order to ensure that each file server has the exact samedirectory structure, for each directory create request received from theclient, the aggregator must create the specified directories on all thefile servers. The aggregator (i.e., the file switch) extracts thedirectory portion of the file path and creates the same directorystructure on all file servers in parallel.

[0327] In case the network file system semantics allows creating fileswithout having to pre-create their directories, the file switch createsthe directories on all servers (at least all servers that can be used tostore metafiles), regardless of where the metafile is created. As anexample, if the file path being created is “\eng\doc\archive\mydoc.doc”,the aggregator must create the parent directory “\eng\doc\archive” onevery file server in the appropriate NAS array where the file“mydoc.doc” is to be stored.

[0328] Load Balancing at the File Switch Level

[0329]FIG. 18 illustrates a mechanism provided by the present inventionfor load balancing at the file switch level. Since all file switcheswithin an aggregated file switch provide access to the same set offiles, any client may be connected to any of the file switches. Thisallows clients to be distributed among the file switches so that not allclients are connected to the same file switch. This can be achieved bymanually configuring each client to use a particular file switch or byautomatically distributing the clients when they try to connect to theaggregated file switch 1803.

[0330] The selection of which particular file switch is going to serve agiven client happens when the client connects to the file switch. Thisassociation preferably does not change for the duration of the clientconnection.

[0331] The load distribution is preferably done through a nameresolution service, such as DNS or WINS, that provides a mapping betweena name (configured as server name for the clients) and the IP address ofa particular file switch.

[0332] One possible mechanism is to have the group 1803 be assigned aseparate DNS subdomain (e.g., zx1.z-force.com). File switch 1801, whichis configured as a group controller also acts as a DNS server for thatsubdomain. The subdomain preferably contains two host names, such asadmin.zx1.z-force.com and zx1.z-force.com. The nameadmin.zx1.z-force.com is used for management, the host namezx1.z-force.com is used for file serving (i.e., this is the name towhich clients connect). The group controller always resolves theadmin.zx1.z-force.com host to itself. It resolves the zx1.z-force.comhost name dynamically. In different embodiments, the zx1.z-force.comhost name is resolved to a respective file switch on a rotating basis, arandom basis, on the basis of the number of users connected to each ofthe file switches, or on the basis of the current transactional loadsbeing handled by the file switches in the group (the file switches mayreport their load factor periodically to the group controller 1801). Asa result, different clients end up on different switches. Each of theswitches may also have a unique name in the subdomain (e.g.,switch3.zx1.z-force.com).

[0333] In an alternative embodiment, the group controller can be adedicated device instead of the file switch 1801.

[0334] Another mechanism for load balancing is for each file switch tohave a different server name and IP address. The system administratorcan configure different groups of clients to connect to different fileswitches (e.g., based on company structure), or use a third-party loadbalancer or round-robin DNS such as RRDNS.

[0335] Yet another mechanism that can be used by the file switchesbelonging to the same group is to configure the switches with the sameserver name (e.g., the CIFS server name), and have that name registeredas a group name instead of an individual host name. When a client triesto establish a connection to that name, the first switch able to respondwill get the client connection. Since typically this will be theleast-loaded switch, this mechanism can also be used for load balancing.

[0336] One skilled in the art will recognize that other mechanisms canbe used to achieve load balancing. One skilled in the art will alsorecognize that combining a load-balanced front end with independentconnections on the back end of the file switch allows practicallyunlimited scaling up of the bandwidth of the network file system, simplyby adding file switches to the group 1803. In such case, one may alsoincrease the number of file servers to which the file switches connectas needed to achieve the desired aggregate performance.

[0337] Transaction Aggregation

[0338]FIG. 19 illustrates transaction aggregation by a file switch. Fileswitch 200 receives a file read request 1901 from a client connectedthrough connection 209. The switch determines the subset of file serverson which instances of the aggregated file reside, preferably by usingthe aggregation descriptor 803 for that file (as described in thefollowing section); in this example, servers 201, 202, 203 and 204,collectively identified as the file array 1900. The switch then submitsappropriately modified file read requests 1902, 1903, 1904 and 1905 toservers of the file array 1900, in parallel. The servers 201 through 204receive their respective file read requests 1902 through 1905, executethem in parallel and respond according to protocol back to the switch,each believing that the switch is its client for the individual filethat resides on that server. The file switch 200 collects all responsesfrom the file servers. Next, it updates its state with informationregarding the member files that comprise the aggregated file, eachresiding on one of the servers 201 through 204 of the file array 1900.Then it aggregates the transaction result and submits it back to theoriginal client.

[0339] As a result, the client can now initiate various filetransactions on the file (in this example, FILE1), as if it were asingle file residing on a single file server. The switch aggregatesdifferent transactions differently. Its operation on read and writetransactions is described elsewhere in this document. The operation ofthe file switch with respect to concurrency-related requests and issuesis described in the following section.

[0340] Accessing an Aggregated User File Through the Metafile

[0341]FIG. 20 illustrates the preferred method for accessing anaggregated user file through the metafile. Upon receiving a fileoperation request from a client, the file switch follows similarpatterns without regard to the actual command being processed. Themethod starts in block 2002 and goes through the following steps.

[0342] In step 2004, the metafile is accessed to fetch the metadata ofthe user file. The location of the metafile is determined by applying anamespace rule to identify a NAS array (i.e., a group of file servers)and by applying a hash function to the given user file name and thegiven file path to identify a particular file server within theidentified NAS array.

[0343] In step 2006, the file server that stores each individual datafile of the user file is determined from the metadata of the user file.Treating the set of file servers in which the data files are stored as a“file array,” each file access operation is executed over a specific setof data files in the file array.

[0344] In step 2008, the file aggregator submits the file accesscommand(s) to the selected file array (or a subset thereof). Thecommands are preferably submitted to the different file array memberssimultaneously (or in quick succession), so that all members willreceive them practically at the same time.

[0345] In step 2010, the file aggregator (i.e., the file switch) waitsand receives response(s) from the selected array of file servers. Afterall operations are submitted to their recipients, the file aggregatorwaits for a response from each of the array elements participating inthe command. The responses may come in any order at any time. It is notnecessary for the file aggregator to wait until the entire and completeresponse is received from a file array member. Once the file aggregatorreceives enough of the response in order to make a decision about thesubmitted command, it may stop waiting for the response from thatmember.

[0346] In step 2012, the file aggregator computes the aggregated result.When all the file array member responses are received, the fileaggregator combines them in an aggregate response.

[0347] In step 2014, the file aggregator submits a response back to theclient. After all responses are received from the file array members andthe aggregate result is calculated, the final response is sent back tothe client. Each of the client's operations are preferably executedasynchronously due to the fact that the file aggregator preferablysubmits each command to the file array members across a network.Finally, the method ends in block 2016.

[0348] General Algorithm for Handling Client Accesses

[0349] This section presents the general aggregation algorithms used toaggregate operations over metafiles in an aggregated file system. Thereare two general algorithms: 1) perform operation over all metafiles forthe user file, and 2) perform operation on a single metafile. Whichalgorithm is used is mostly dependent upon the type of file operationexecuted.

[0350] Perform Operation Over All Metafiles

[0351] In this algorithm, operations are executed over all metafiles fora given user file. One case this algorithm is used is for all operationsthat modify the metadata stored in the metafiles. For example, thisalgorithm is used when creating files for access, and when deletingfiles. The operation is repeated over all metafiles in parallel forhighest performance.

[0352] Note that the operations are performed only over metafiles thatreside on currently available servers. If one of the copies of themetadata is not available, the modifications are stored in the others;at least one copy must exist in order for access to be provided.

[0353] Perform Operation Over a Single Metafile

[0354] This algorithm is preferably used for non-destructive fileoperations that retrieve but not modify data in the metafile, such asgetting the last modified time of the file. In this algorithm, anoperation is performed over the metafile stored in the metaserver withthe lowest ordinal number. Alternatively, the operation may be performedover the metafile stored in a randomly or pseudo-randomly selectedmetaserver, from among the metaservers currently believed to beavailable.

[0355] Handling Concurrent Accesses

[0356] Since file servers and network file protocols are designed foraccessing by multiple clients simultaneously, they typically provideexcellent support for concurrency handling. For example, the CIFSnetwork file protocol provides the ability to request an exclusive fileopen, meaning that if two clients request open at the same time, onlyone of the requests is going to succeed.

[0357] In the case of a single file server, this support is oftenimplemented inside the file server by using operating systemsynchronization objects. This works well for a single server in whichaccess from multiple clients can be serialized within the same computer.However, as the background discussion explains, extending this approachto multiple servers in a clustered configuration creates a bottleneck.For this reason, the present invention preferably uses a differentmechanism for handling concurrency.

[0358] An Exemplary Concurrency Problem

[0359]FIG. 21 illustrates an exemplary concurrency problem when twoclients trying to access the same resources simultaneously. The system2108 consists of two file switches 200 and 2106, file servers 201through 207, and a layer 2 switch 2107, which is used to connect thefile servers and the file switches.

[0360] In this example, two clients send requests for a file writesimultaneously. A first client, client A is connected to file switch 200and sends its file write request 2111 to it; a second client, client Bis connected to the file switch 2106 and sends its file write request2101 to it. In this example, the requested file is aggregated from fourdata streams (e.g., four mirrors), each residing on one of the servers201 through 204 (the four servers forming the file array 2100 for thisfile).

[0361] Both file switches process the request at the same time and tryto process it by switching the incoming requests 2111 and 2101 to eachof the four servers of the file array 2100. File switch 200 sendsrequests 2112 through 2115 to the file servers 201 through 204,respectively. File switch 2106 sends requests 2102 through 2105 to thefile servers 201 through 204, respectively. While the two switches mayhave issued the requests at the same time, the requests arrive at eachof the file servers in some order. In this example, the file servers201, 203 and 204 receive the requests 2112, 2114 and 2115, respectively,before they receive the corresponding requests from the file switch2106, namely the requests 2102, 2104 and 2105. However, the file server202 receives the request 2103 from the file switch 2106 before itreceives the request 2113 from the file switch 200. One skilled in theart will easily recognize that several other orders are possible, aswell as similar situations with more than two clients, more than twoswitches and another number of file servers.

[0362] Based on the above-described order of arrival of requests, thefile servers 201, 203 and 204 satisfy the write requests 2112, 2114 and2115 coming from file switch 200 (data A) while the file server 202satisfies the request 2103 from the file switch 2106 (data B). Themirrors of the file contain inconsistent data as a result of theconcurrent accesses by both client A and client B without proper lockingmechanism. From the standpoint of a file switch, both aggregatedtransactions will fail, since neither of them would succeed in writingall four of the member files. This scenario is clearly in violation ofthe semantics of the write request, which requires that one clientshould succeed and all others should fail.

[0363] One skilled in the art will recognize that this situation canoccur with other operations. For example with a lock request, thissituation leads to the classic deadlock problem. Although the resourcethat both clients requested (i.e., the aggregated file) is available andcan be granted to one of the clients easily, none of the clients is ableto acquire it (i.e., write to the file). The concurrent access problemdescribed above, with respect to write operations, can be solved usingimplicit locking, as described next. Concurrency problems associatedwith the open-exclusive operation and with lock requests are solvedusing mechanisms described below with reference to FIG. 24.

[0364] Implicit Locking

[0365] Network file protocols typically provide file-level locking andbyte-range locking in order to synchronize multiple clients that try towrite to the same file and the same area within a file. When locking isused consistently by all clients, there is no need for additionalsynchronization in order to avoid inconsistent data being written todifferent mirrors of the same file; however, not all file clientapplications use the locking mechanism consistently.

[0366] Implicit locking allows a client to write data into a locked byterange while sharing the same file with other clients. While a clientholds a lock on a byte range in a file, it is the only client that isallowed to write data into that portion of the file. Other clients cannot read or write data in the locked range area. This gives a client anexclusive access to a specific portion of the file but not to the entirefile. If byte range locking is used consistently by all clients, thereis no need for additional synchronization in order to avoid inconsistentdata being written to different mirrors of the same file. However, notall client applications use the locking mechanism consistently, whichcan result in data corruption in an aggregated file system.

[0367] Another application of implicit locking is when the fileaggregator needs to lock a portion of the file if a client is trying towrite data to the file and does not have exclusive access to the targetarea of the file. The file aggregator (i.e., the file switch) isconfigured to lock the corresponding byte range of a file if the clientattempts to write data into the file without first locking the rangeitself; the aggregator locks the byte range on behalf of the client. Theaggregator preferably locks the byte range if the client does not haveexclusive access to the whole file or exclusive access to the accessedportion of the file in which it intends to write. When the writeoperation is complete, the file aggregator unlocks the previously lockedbyte region of the file (if it had locked it implicitly).

[0368]FIG. 22 illustrates a method for implementing implicit lockingwith metafiles that ensures that a client writing to a file hasexclusive access to that portion of the file and keeps all mirroredcopies of the file properly synchronized with the correct data. Themethod starts in block 2200 and then moves through the following steps.

[0369] In step 2202, the file aggregator receives a file write requestfrom a client. Typically before issuing a write request, a clientpreferably requests, through the file aggregator, a byte range lock ofthe section of the aggregated file to be modified. Next, the fileaggregator forwards the client's byte range lock request to theappropriate file servers in the correct NAS array. Then, the fileaggregator gathers the byte range lock results from the file servers,forwards the aggregated result back to the client, and saves the stateof the specific byte range that has been locked by the client. However,the procedure shown here does not assume that a byte range lock hasalready been obtained.

[0370] Upon receiving the client's write request, the file aggregatorfirst determines, in step 2204, whether the byte range of the writeoperation has been locked by the requesting client. If the byte range islocked, the method moves on to step 2214.

[0371] In the alternative, if the byte range is not locked, then themethod moves to step 2206 where the file aggregator generates byte rangelock requests to each of the file servers that contain a copy of thefile on behalf of the client. In one implementation, the byte range lockrequest is forwarded to the appropriate file servers so as to requestlocks on the data files containing the data in the specified byte range.To do this, the primary metafile for the specified user file is firstaccessed to determine the identities and locations of the data files forthe stripes containing the specified byte range. Then the lock requests,for locks on the required portions of those data files, are forwarded tothe appropriate file servers.

[0372] In a second preferred implementation, the byte range locks areobtained on the primary metadata file and its copies; no locks areobtained on the underlying data files. In particular, a byte range lockmay be obtained on a file, such as a metafile, even when the byte rangespecified in the lock request is partially or even completely outsidethe range of data actually stored in the file. Thus, in thisimplementation, the byte range lock requests are directed to all thecopies of the primary metadata file, corresponding to the user file onwhich the lock has been requested (whether explicitly or implicitly). Toprevent deadlocks, the byte range lock request is first directed to theprimary file server for the metafile (as determined, for example, by ahash function or other selection function); and after the lock requestis granted by the primary file server, the same lock request is thendirected to the other file servers on which copies of the metafile arestored.

[0373] Both implementations utilize the lock management capabilities ofthe file servers, with the primary roles of the file switch being theapplication of the namespace rules to determine the file servers towhich the lock request should be directed, replication of the lockrequest to those file servers, and aggregation of the lock requestresults. In yet another implementation, the aggregation rule applicableto the user file includes a lock redundancy parameter P that specifiesthe number of primary metafile copies on which the lock is obtained. Inthis implementation, the lock request is directed to a primary fileserver for the metafile, and then to P−1 other file servers, selected ina predefined manner (e.g., based on ordinal numbers associated with thefile servers, using a round robin selection function).

[0374] In step 2208, the file aggregator gathers the byte range lockresults from the file servers and save the state of the specific byterange that was locked. In step 2210, a determination is made as towhether the byte range lock has been acquired by the file aggregator. Ifthe byte range lock has been acquired, the method continues in step2214. If the byte range lock has not been acquired, then the fileaggregator fails the write request and sends a notice to the client instep 2212.

[0375] In step 2214, after confirming the client has secured the byterange lock either in step 2204 or step 2210, the file aggregatorperforms the write operation to all file servers that contain theaggregated file. In step 2216, the file aggregator receives andaggregates write responses from the file servers. The method then movesto step 2218 where the file aggregator sends an acknowledgement to theclient when the write operations have successfully completed.

[0376] In step 2220, the file aggregator releases the byte range lock.This step is performed regardless of whether the write operations havecompleted successfully as in step 2218 or the write request has failedas in step 2212. After releasing the byte range lock, the method ends inblock 2222.

[0377] When this mechanism is consistently used by the file switch, andin the case of multiple file switches accessing the same set of fileservers by all file switches, it ensures consistency of the data file ata level comparable to that maintained by any single file server.

[0378] Opportunistic Locks and Caching

[0379] Another mechanism frequently deployed with network protocols isOpportunistic Locks (“oplocks”; also known as callbacks). Oplocks allowclients to cache the data file locally to increase performance whilekeeping the files synchronized and consistent. Depending on the networkfile system that is used, oplocks may or may not be supported and thedifferent types of oplocks may vary. Most existing operating systems,including Microsoft Windows and LINUX (e.g., SAMBA), support oplocks.

[0380] Oplocks are usually only requested by a client when the clientopens a file on a network file server. When requesting an oplock, aclient always requests an oplock. If the oplock is granted to a client,the client may then cache data file locally to increase performance. Ifan oplock is not granted, the client must send all network file requestsover the network and it can not cache any data from the file. A serverdoes not have to grant the oplock specified by the client; it may grantthe client a different level of oplock than the one requested.

[0381]FIG. 23a illustrates a method for handling an oplock request by aclient. The method starts at step 2300 and continues to step 2301 wherethe file aggregator (i.e., a file switch) receives the client's requestof an oplock to a user file. In step 2302, the aggregator sends oplockrequests on the metafiles corresponding to the specified user file to apredetermined array of file servers. Next, the aggregator waits andaggregates the responses from the file servers (step 2303) and grantsthe client the lowest level oplock that was granted by the servers forthe metafiles (step 2304). Note that oplocks are used on metafiles only,not on data files. Then, in step 2305, the aggregator saves the state ofall the granted oplocks from the file servers. In step 2306, the oplocklevel granted to the client is also saved as the current oplock levelfor the file aggregator. The method ends at block 2307.

[0382] Oplocks can be “broken” at any time. This means that after aserver grants a specific oplock to a client, the server can send anotification that tells the client that it no longer has the right tohold its current oplock. This usually occurs when a second client triesto open the same file. The server may downgrade the current oplock to adifferent oplock or may remove the oplock completely from the client.Depending on the new oplock granted by the server, the client may haveto flush any cached data file back to the server to keep the filesynchronized with other clients. If the client no longer holds an oplockon the file, all cached data file must be flushed and all subsequentfile operations must be sent over the network to the file server.

[0383]FIG. 23b illustrates a method for handling oplock breaknotifications from a file server. The method starts at step 2310 andcontinues at step 2311 where an oplock break notification from a senderfile server is received. Then, in step 2312, the file aggregator (i.e.,the file switch) compares the level of oplock break notification fromthe file server versus the oplock level granted to the client.

[0384] In step 2313, if the level of oplock break notification is lowerthan the oplock level granted to the client, the forwards the oplockbreak notification to the client. Then in step 2314, the aggregatorwaits for the client to respond to the oplock break notification, andupdates the current oplock level to the new oplock level. In step 2315,the aggregator forwards the client's response to the file server thatoriginated the oplock break notification.

[0385] In step 2316, if the oplock break notification specifies anoplock level that is equal to or greater than the current oplock levelthat was granted to the client, the aggregator responds to the oplockbreak notification. It then updates its state to reflect the new oplocklevel for this file server in step 2317. Since the client may hold anoplock that is lower than the oplock specified in the notification,there is no reason to propagate the notification to the client.

[0386] In step 2318, if the client never requested an oplock when itopened the file or does not hold an oplock associated with this file,the aggregator responds to the oplock break notification.

[0387] It then updates its state with the new oplock level in step 2319.The method ends in step 2320.

[0388] Note that, before responding to any oplock break notificationreceived from a file server, the aggregator (i.e., file switch) mustfirst update any oplock state as necessary. As a result, data cachedwithin the aggregator may need to be written back to the file server, ifthe cached data has been modified, and cached data in the aggregator mayneed to be invalidated if the oplock is being totally withdrawn by thefile server. If multiple oplock break notifications are received fromdifferent file servers around the same time, they are queued and handledone at a time. In addition, it is not necessary to respond to theserver's oplock break notification if the client chooses to close theaggregated file when it receives the notification from the aggregator.Some network file systems accept a file close operation as a response toan oplock break notification.

[0389] There are several different types of oplocks that can be granted.The types of oplocks are defined by the network file protocol that isused with the file aggregator. The type of oplock defines exactly howthe client can cache data, ordered by the level of caching given to aclient. FIG. 23c illustrates a method for mapping a level of exclusivityof caching to the oplock exclusivity level granted. For example, whenusing the CIFS file protocol, an “exclusive” oplock allows the client2330 to cache a data file “myFile.doc” 2331 locally. Under an exclusiveoplock, all read and write operations can be executed locally andtherefore the file access time is reduced. A “level 2” oplock allows thedata file “myFile.doc” 2333 to be cached in the file switch 2332 or inthe client. A level 2 oplock allows all clients given this level ofoplock to cache read data locally. (The oplock is revoked the first timesomeone writes to the file). Note that the file switch can also use theoplock level in order to determine whether it can cache read data, inaddition to or instead of the clients. This file is shared among clientssupported by the file switch 2332. “No Oplock” is the lowest level,where the client is not allowed to cache the file “myFile.doc”. Under“no oplock”, mirrors of this file 2335 and 2337 are stored in the fileservers 2334 and 2336 respectively.

[0390] In an alternate embodiment, oplocks requests are directed to andhandled by the file servers that store data files for a specified userfile, instead of being handled by the file servers that store themetafile for the specified user file. The file switch distributes theoplock requests to the file servers accordingly, and also aggregates theoplock responses, break messages, and so on from the same file servers.The number of file servers to which each oplock request is directed isdetermined by the number of stripes that are included in the subset ofthe file for which an oplock is being requested, and the level of lockredundancy to be used. This method allows the file switch to cachefragments of the file differently on different file servers.

[0391] In one embodiment, implicit locking is used in combination withopportunistic locking. In particular, when a client does not request anoplock in conjunction with an operation on a user file, the file switchmay nevertheless request an oplock from the file servers when predefinedimplicit locking criteria are met (e.g., when the nature of the clientrequest, or a usage pattern by the client, indicates continued access tothe file is likely). When the implicit oplock is granted, the fileswitch preferably caches data from the file specified by the client,without the client having any knowledge that such caching is occurring.By opportunistically caching data in the file switch, the file switchprovides faster access to data in the specified file. This can beespecially helpful when the file switch is much closer to the clientcomputer than the file servers on which the requested file resides. Inaddition, while the file switch caches data from a file, it can respondto requests from more than one client requesting data from that file,using the same cached data to provide fast responses to each of theclients, so long as none of the clients requests exclusive access to thefile.

[0392] In some embodiments, the file switch can cache data and use thecached data to provide fast response to two or more clients or clientcomputers, even when one or more of the clients have requested an oplockon the same file. In other words, when a second client attempts toaccess the same file for which an oplock has been granted, the oplock isnot necessarily broken. Rather, if the accesses by all the clients arecompatible, then the file switch caches the oplock state (if any)associated with each client requesting access to the same file, andsends responses to the clients using the cached data from the file. Thecaching of the data in the file switch ends when caching terminationcondition arises, such as a client requesting exclusive access to thefile, or all clients closing the file.

[0393] Semaphores

[0394] A semaphore is a mechanism that allows only a certain number ofentities to access a particular resource. In the context of anaggregated file system, a semaphore is used to allow only one fileswitch to access a specific aggregated file at a time. This includes alloccurrences of the file on all file servers in the NAS array (i.e., ifthe file is striped or mirrored among multiple file servers). In anaggregated file system, the semaphore is achieved using the primarymetadata file stored on the NAS arrays as the semaphore object. Theprocess that obtains access to the primary metadata file also obtainsaccess to the aggregated user file as a whole (the file may still beshared among multiple clients).

[0395] The semaphore synchronization mechanism is used mainly withdestructive file operations. Destructive file operations includecreating a new file, truncating an existing file, deleting an existingfile and renaming or moving an existing file. The semaphoresynchronization mechanism is also used with non-destructive operations,such as exclusive open.

[0396] Synchronization is needed for destructive operations sinceexecuting the operations over a specific file changes some aspect of thefile; if the aggregator needs to back out and let another entity haveaccess to the same file, it would have to restore the state of all filesthat it accessed. This would require keeping the states of thetransactions on the file switch, which is very costly and can degradeperformance. By using the semaphore synchronization mechanism, anaggregator does not execute destructive file operations over any filesunless it is granted access to the files by way of a semaphore.

[0397]FIG. 24 illustrates a method for handling concurrent accessesusing a semaphore. The method starts in step 2400 and moves to step 2401where the file aggregator receives a request for opening the file forexclusive access (not a destructive operation). Also in step 2401, thefile aggregator determines the location of the primary metafile of therequested user files by applying a hash finction on the user file name.

[0398] Next, in step 2402, the file aggregator tries to open the primarymetafile with exclusive file access and no file sharing allowed. In step2403, a first determination is made as to whether the primary metafilehas been successfully opened. If the answer is positive, the methodcontinues in step 2405. If the answer is negative, the file aggregatorfails the client's file access request and moves to step 2409; or waitsa random amount of time and retries to open the primary metafile again.There should be a limit on the number of retries. If opening themetafile has succeeded, the aggregator is granted access to theaggregated file. If there is more than one copy of the primary metafile,then the open is considered successful if all opens completedsuccessfully; if at least one open failed indicating that the file isalready open, the client's request for exclusive open will be denied.

[0399] In step 2405, the file aggregator opens all the data streams onall of the file servers of this user file's file array, or alternatelyopens all the data streams that will be needed for the destructive fileoperation. Step 2405 ensures that all the data streams required for thedestructive file operation are available.

[0400] In step 2406, a second determination is made as to whether allopen requests have been granted by the file servers. If any of the openrequests fail, the file aggregator fails the client's file accessrequest in step 2407 and moves to step 2409. In the alternative, if allopen requests have been granted successfully, the method moves to step2408 and the file aggregator performs file access on all data streamfiles. In step 2409, after all the file accesses have been completed,the file aggregator closes all the data files and then closes theprimary metafile(s). The method ends in step 2410.

[0401] With each aggregator accessing the files using this methodology,it can be guaranteed that the access to the file will be properlysynchronized.

[0402] Summary of Aggregation of Concurrent Accesses

[0403] One skilled in the art will recognize that other algorithms maybe employed to achieve the same results and ensure consistent and atomicbehavior for aggregated transactions. Similarly, one skilled in the artwill recognize that the same approaches may be applied to other filetransaction types, such as locking, creation, etc.

[0404] In effect, the present invention aggregates the existingsynchronization mechanisms provided by network file protocols (and thusby the file servers in the system) to implement synchronization betweenthe clients of multiple independent file switches without requiringdirect interaction and communication, and therefore, coupling, betweenthe file switches. In addition, each individual file switch can furtheruse these mechanisms in order to synchronize transactions requested bymultiple clients that are connected to that switch.

[0405] Directory Enumeration

[0406] When a file switch receives a directory enumeration request froma client, the request may specify to enumerate an entire directory (notincluding sub-directories) or it may enumerate a single file. Singlefile enumeration is typically used to determine whether or not aspecific file exists in the file system. This section covers how toenumerate a single directory or a single file.

[0407] When a directory enumeration request is received, the aggregatedfile system uses the namespace aggregation rules to determine which NASarrays need to be enumerated in order to satisfy the request. Anyparticular directory (i.e., an aggregated directory in the usernamespace) may be distributed over multiple different NAS arrays becausemultiple namespace rules may apply to the files in that one directory.The file aggregator enumerates the corresponding directories on all theNAS arrays that are the target of the applicable namespace rules,combines the results, and propagates the combined result back to theclient.

[0408] When enumerating the directories in an aggregated file system,all of the file servers of a specific NAS array are preferablyenumerated for their directory contents. This is due to the fact that ahash function distribution function is used to distribute the metadatafiles among different file servers of the NAS array. Only the metafilesare enumerated; data files are ignored. The main goal of the aggregateddirectory enumeration mechanism is to efficiently eliminate duplicatefiles in the enumeration so that aggregated directory enumeration isfast and efficient.

[0409] The basic aggregated directory enumeration method is as follows.When a file switch needs to enumerate a directory on a NAS array, theclient's enumeration request is replicated in parallel to all of thefile servers in the NAS array. The file switch receives all of theresponses from the servers and builds the enumerated directory structureentirely in memory. The file switch does not wait for the entiredirectory structure to be built in memory before sending enumerationresults back to the client. Rather, the enumeration results are sentback to the client as soon as they are available.

[0410] The directory enumeration strategy is defined in the followingtwo sections:

[0411] Enumeration State: Describes the internal state that the fileswitch needs to maintain during a directory enumeration operation.

[0412] Enumeration Algorithm: Defines the algorithm of how to enumeratea directory over a set of NAS arrays.

[0413] State Information Related to the Directory Entries

[0414] In order to enumerate the directories on a NAS array, theenumeration request is sent to all file servers of the array and theresponses are collected. Since the enumerated directory structure isbuilt entirely in memory from these responses, the file switch needs tomaintain the following internal state (i.e., the enumeration state):

[0415] a list of directory entries;

[0416] additional state related to the directory entries; and

[0417] a list of pointers to the directory entries.

[0418] A List of Directory Entries

[0419] After the enumeration request is replicated to all file serversof a NAS array, the file switch collects all of the responses. Theseresponses contain a list of files that are contained in the enumerateddirectory. The responses should contain only listings of primary andsecondary metafiles, because data files are stored in a differentsub-tree on the file servers. For each listed file, the responsecontains the directory information requested in the enumeration request,such as file name, file size, and other file attributes. Each filelisting returned in the enumeration set is known as a directory entry.

[0420] Each file found in the enumeration response is added to alist/array of directory entries maintained in memory in the file switch.In a preferred embodiment, each directory entry is added to the list inthe order in which it is received and processed. The list or array ispreferably implemented as either a queue or a linked list.

[0421] Each distinct user file must appear in the final enumerated listonly once. Duplicate file names refer to files with the same name thatare located in the same user namespace directory. Duplicate files mayappear because the file switch replicates the metadata files forredundancy.

[0422] Additional State Relate to the Directory Entries

[0423] For each directory entry, there is additional state that istracked by the file switch during enumeration. This state includes thefollowing:

[0424] The number of times the file was found in the enumeration(duplicate files). This occurs since metadata files are replicated forredundancy. Separate counters are maintained for the primary andsecondary metafiles.

[0425] Whether or not the file has been submitted back to the client aspart of the directory enumeration response.

[0426] The additional state can be kept as part of the directory entryarray or can be stored in a separate array.

[0427] A List of Pointers to the Directory Entries

[0428] For each directory entry that is processed by the file switch,the file switch must search the directory entry list to see if the fileis already included in the list. This can be a very time consumingprocess, especially if the directory entry list contains thousands ofunsorted entries.

[0429] In order to speed up the enumeration process, the file switchmust maintain a list or array of memory pointers that point to specificentries in the directory entry array. The pointer list contains pointersto the directory entries ordered alphabetically. Using the pointer list,the file switch can quickly search through the directory entries using abinary search to find out whether or not a file exists in the directoryentry list. If a new file needs to be added to the list, the file switchonly needs to update the pointer list and no entry data needs to becopied in memory.

[0430] Directory Enumeration Algorithm

[0431]FIG. 25 illustrates directory enumeration for the aggregated filesystem. During directory enumeration, directory requests are sent toredundant directories of metafiles and duplicate responses are filteredout. This is done to ensure that if a file server fails while processinga directory enumeration request, the directory enumeration request isprocessed to completion using data obtained from the other file servers.The directory enumeration request is processed just as quickly as if thefile server had not failed. Thus, the directory enumeration method makesindividual file server failures invisible to the client. Only if thereis a failure of all the file servers on which redundant metafiles arestored will directory enumeration service to the client computers beimpacted.

[0432] The method starts in step 2500 and then moves to step 2501 wherethe file switch receives a directory enumeration request (e.g., acommand asking for a listing of all files in a particular directory)from a client.

[0433] In step 2502, given the directory to be enumerated, the fileswitch determines the set of NAS arrays that need to be enumerated basedon the namespace aggregation rules and the directory path beingenumerated. More particularly, the file switch determines, from thedirectory path specified in the request, all namespace rules that areapplicable to the request. Those rules specify the NAS arrays that storethe files in the specified directory path. Each NAS array is enumeratedin exactly the same way. The file switch may enumerate the NAS arraysone at a time. When the enumeration is completed on one NAS array, thefile switch moves to the next NAS array (if any) using the same internalstate information.

[0434] Once the set of NAS arrays is determined, each NAS array isenumerated one at a time. Step 2503 marks the beginning of the controlloop for processing directory information for each NAS array identifiedin step 2502. In step 2503, the file switch extracts the match pathportion after the last backslash ‘\’ of the enumeration path (e.g.,“*.*”, “*.doc”, or “a*.doc”). If the first character of the match pathis not the wildcard character “*”, the single character wildcard “?” isadded as a prefix to the match path. If more than one NAS array isidentified in step 2502, the match path portion of the enumeration path(extracted in step 2503) is different for each identified NAS arraybecause each stores only a portion of the files in the directory to beenumerated. For example, a first particular NAS array identified in step2502 may only store files (in the specified directory) having a fileextension of “doc”. If the directory enumeration request is for filesstarting with the letter “a” (e.g., dir a*.*), the extracted match pathportion for this first NAS array would be “?a*.doc”. In otherembodiments, where there is no secondary metafile, the metafile has thesame name as the user file, so the extracted match path will not needthe “?” prefix.

[0435] The extracted match path portion is used by the file switch instep 2504 to retrieve all of the metafiles that match the match pathportion. In the simplest case, if the enumeration path specifies only asingle file with no wildcards (e.g., “dir file1.doc”), the file switchsimply replicates the request to the appropriate set of file servers ofa single NAS array, with a “?” wildcard prefixed to the filename. Theresponses are collected and a consolidated response is sent back to theclient. No other steps are executed. The directory entry list, pointerlist and additional state information are emptied or reset to contain noentries.

[0436] More generally, in step 2504, the file switch replaces theenumeration path according to the namespace aggregation rules (i.e., asdetermined in step 2503) applicable to the NAS array currently beingprocessed, and replicates the enumeration request in parallel to all ofthe file servers in the NAS array that are configured to store metadatafiles. In some embodiments, the NAS array is configured so that some ofthe file servers in the NAS array store metadata files, while other fileservers are configured to store data files (i.e., files other thanmetadata files); in other embodiments, some file servers may beconfigured to store both metadata files and data files, while other fileservers are configured to store only data files. In step 2505, the fileswitch waits and receives the responses to the enumeration requests fromthe file servers.

[0437] Step 2506 marks the beginning of the control loop for processingthe response received from each file server. In step 2506 a first ornext file name in the response received from a file server is processed.The file switch searches the pointer list by file name to see if thefile name is already included in the directory entry list. During thisstep, the ‘P’ or ‘S’ prefix of the file name, which indicates whetherthe listed file is a primary or secondary metafile, is stripped from thefile name for purposes of searching the pointer list.

[0438] In step 2507, a determination is made as to whether a new entryhas been received. If the entry is not new, i.e., the file exists in thedirectory entry list built in memory, then the method takes the NObranch and moves to step 2509 where the file switch updates the stateand pointer related to the existing directory entry. The state of thedirectory entry includes the directory information returned by the fileserver with the directory entry. In step 2509, the file switch alsoupdates the additional state of the directory entry with the number oftimes the primary and secondary metafiles have been found.

[0439] In the alternative, if the entry is new, then the method takesthe YES branch and moves to step 2508 where the file switch adds thedirectory entry to the directory entry list and initializes the state ofthe new directory entry. The filename used in the directory entry doesnot include the ‘P’ or ‘S’ prefix of the primary or secondary metafilerepresented by the received filename. The file switch also updates thepointer list with a pointer to the new directory entry in the properalphabetical order and initializes any other additional state needed forthe new entry.

[0440] In step 2510, a determination is made as to whether both theprimary and secondary metafiles for a user file have been found. If theprimary and secondary metafiles have not been found according to thefile aggregation rules, the file switch does not send the directoryentry back to the client that requested the directory enumeration,because it does not yet have sufficient information to send back to theclient. Instead, the method moves to step 2512 and continues with thenext entry returned by the file servers (at step 2506). In thealternative, if both the primary and secondary metafiles have beenfound, the directory entry contains all the requested directoryinformation for the corresponding user file, and this directory entry issent back to the client. The directory information for the secondarymetafile contains the aggregated file size and allocation size of theuser file. The directory information for the primary metafile containsall other file information, including access/creation dates and times,file attributes and so on. (An alternate embodiment that changes theoperation of step 2510 is discussed below.)

[0441] In step 2511, the file switch submits the entry back to theclient as part of the enumeration response. The file switch preferablyuses a different thread to submit an entry back to the client. Thisthread runs in parallel with the threads that are enumerating thedirectories on the file servers. If the entry has already been submittedback to the client, the file switch does not return the entry to theclient in step 2511, and instead skips over to step 2512.

[0442] In step 2512, a determination is made as to whether all files inthe directory has been enumerated. If the answer is negative, the NOpath is taken and the method moves to step 2515 before it continues withthe next entry returned by the file server (step 2506). The directoryenumeration continues until all of the files are enumerated and storedin memory. In the alternative, the YES path is taken and the methodmoves to step 2513. Note that if there are any directory enumerationerrors, but at least one of the enumeration requests to the file serversis successful, a positive enumeration response is sent back to theclient with the collected enumeration results. If all of the enumerationrequests fail, the client's enumeration request fails and a failureresponse is returned to the client.

[0443] In step 2513, a determination is made as to whether all fileservers have been enumerated. If the answer is negative, the NO path istaken and the method moves to step 2515 before it continues with thenext file server in the file array (at step 2504). In the alternative,the YES path is taken and the method moves to step 2514 where anotherdetermination is made as to whether all NAS arrays have been enumerated.If the answer is negative, the NO path is taken and the method moves tostep 2515 before it continues with the next NAS array in the switchedfile system (at step 2503). In the alternative, the YES path is takenand the method ends in step 2516.

[0444] In step 2515, a termination condition is checked as to whetherthe client has closed the enumeration. If the termination condition hasnot occurred, the method continues at step 2503, 2504 or 2506, dependingon the iteration loop the method is in, as indicated by which step wasperformed (namely 2512, 2513 or 2514) prior to step 2515. In thealternative, if the termination condition has occurred, the YES path istaken and the method ends in step 2516.

[0445] Note that when enumerating directories, the total number ofentries that are in the enumeration set may exceed the number of entriesthat can be returned back to the client due to limitations of theclient's response receive buffer. If this situation occurs, the fileswitch sends an enumeration response containing a subset of the entrieswith an indicator that indicates there are more entries in theenumeration. This enables the client to send another enumeration requestto retrieve the remaining entries.

[0446] When updating the directory entry list of an existing entry,several entry attributes need to be updated (see step 2509 above). Themost important attribute is the size or allocation size of the file. Foreach aggregated file, the size of the file is stored in the secondarymetafile encoded in one of the time/date fields associated with thefile. The allocation size is determined by taking the aggregated filesize and multiplying it by the number of mirrors. All other fileattributes are retrieved from the primary metafile. These attributesinclude last accessed date and time, creation date and time, lastwritten date and time, and so on.

[0447] If after a directory entry is submitted back to the client, thefile switch receives another occurrence of the same file listing on oneof the other file servers, this is not considered an error—becausemetafiles are purposely replicated. In this case, the file listingreceived from the file server is ignored.

[0448] In an alternate embodiment, directory entries are not submittedback to the client at step 2511, but instead a sorted list of directoryentries is built at step 2511. The resulting sorted list is returned tothe client when the building of the list is complete, just before step2516.

[0449] In another alternate embodiment, only a primary metafile isprovided for each user file, and no secondary metafile is used. Asexplained above, one of the directory fields of the primary metafile isused to store the aggregated file size for the corresponding user file.In this embodiment, step 2510 can be eliminated. Instead, step 2508 isfollowed by step 2511, but step 2509 is followed by step 2512. In otherwords, whenever a new metafile is found, its entry is submitted to theclient, but when a redundant metafile is found the directory enumerationprocedure skips over it, except for bookkeeping (step 2509).

[0450] The directory enumeration method shown in FIG. 25 can also beused, with minor modifications, to perform other directory operations(sometimes called file commands), such as changing a specified fileattribute for a specified set of files (e.g., “attrib+r c:\x\y\ab*.doc”)or deleting a specified set of files (e.g., “del c:\x\y\abcd*.doc”). Instep 2504, the file command is sent to the applicable file servers ofNAS server identified in step 2502. Steps 2506 to 2510 are replaced bysimilar steps for aggregating the responses obtained from the fileservers, and in step 2511 or 2516 the aggregated responses are returnedto the client.

[0451] Redundant Metavolume Controller

[0452] A collection of user files is referred to as a “volume” of datafiles. A volume of data files may be stored on one or more file servers,and a file server may host one or more logical volumes. In the contextof the metadata based file switch and switched file system, a collectionof metafiles corresponding to the collection of user files is called a“metavolume”. It is desirable to replicate metavolumes over multiplefile servers to provide backup of the metafiles and to provide continueoperation of the switched file system in event of a failure of one ofthe file servers used to store the metafiles.

[0453] A group of file servers in a NAS array can be designated to storemetafiles. Each such file server is called a metaserver. In someimplementations, all metaservers in a NAS array have identical metafilecontent (i.e., they all store copies of the same metafiles). In otherimplementations, while each metafile is replicated N times on a set ofmetaservers selected using a distribution function, the number ofmetaservers is greater than N, and therefore the metaservers do not haveidentical content. Once a metavolume is created, its configuration (withrespect to the metaserver and directories in which the metafiles arestored) does not change. Each metaserver within the redundant metavolumeis assigned an ordinal number. This ordinal number also does not changeonce a metavolume assigned to the metaserver is created.

[0454] Accessing Redundant Metavolumes

[0455] In general, there are three types of redundant metavolumeoperations: destructive operations, non-destructive operations andcreating new file or lock acquisition. A non-destructive operation, suchas a read operation, does not change the content or attributes of themetavolume, so this operation is performed on any one of themetaservers. On the other hand, a destructive operation, such as adelete operation, does change the content or attributes of themetavolume, and this operation is performed on all the metaservers ofthe NAS array to which the metavolume has been mapped. For creating newfile or lock acquisition, the operation is performed first on theprimary metaserver to obtain the exclusive access to the metavolume, andthen the operation is performed on all other metaservers of themetavolume.

[0456]FIG. 26 illustrates a method for accessing redundant metavolumes.The method starts in block 2600 and thereafter moves to block 2602. Atblock 2602, the redundant metavolume controller (RMC) receives a requestfrom a file aggregator to access the redundant metavolumes stored in agroup of metaservers. In a preferred embodiment, the RMC is implementedas a software module within the aggregated file system 616 (FIG. 6). Inanother embodiment, the RMC may be implemented using one or moreapplication specific integrated circuits (ASIC's), or a combination ofASIC's and software.

[0457] At block 2604, the RMC selects a primary metaserver. The primarymetaserver is selected based on the name of the metafile. In oneembodiment, the RMC selects the primary metaserver by computing a sum Sof all character values of the metafile name and then computer S moduloM, where M is the number of metaservers. The resultant number is used bythe file switch as the ordinal number of the primary metaserver. Inanother embodiment of the present invention, the primary metaserver isselected by computing a hash function of the name of the metafile. Theresultant number of the hash function is the ordinal number of theselected primary metaserver. Both of these methods distribute theprimary metafiles evenly across the available metaservers, and henceimprove the performance of the overall system. In yet anotherimplementation, the primary metaserver is a predefined one of themetaservers, such as the metaserver having the lowest ordinal number.

[0458] At block 2606, a determination is made as to the type of therequested operation. If a destructive operation is requested, the pathto 2612 is taken; if a non-destructive operation is requested, the pathto block 2608 is taken; and otherwise the path to block 2616 is takenfor handling operations such as creating a new file, lock acquisition,rename, and the like.

[0459] At block 2608, the RMC sends the non-destructive operationrequest to the available metaserver with the lowest ordinal number.Alternately, the RMC sends the operation to a randomly orpsuedo-randomly selected metaserver, from among the metaserverscurrently believed to be available. Next, the method moves to block 2610where a determination is made as to whether the metaserver to which therequest is sent is available. If the metaserver is unavailable, the NOpath is taken and the RMC retries the operation to the next availablemetaserver (by the next lowest ordinal number) by repeating the steps inblocks 2608 and 2610. In the alternative, if the metaserver isavailable, the method moves to block 2620.

[0460] At block 2612, the RMC sends the destructive operation request toall metaservers and aggregates the responses from all the metaservers.Next, the method moves to block 2614 where a determination is made as towhether at least one of the accesses to the metaservers is successful.If none of the accesses to the metaservers is successful, the NO path istaken and the RMC fails the destructive operation request. If the accessto at least one, but not all of the metaservers is available and returnssuccess, the operation is considered to have been successfully complete,and the YES path is taken to block 2620. If the destructive operationfails on a particular metaserver, the operation may be retried one ormore times, and if the operation continues to fail, the metaserver maybe denoted as being inoperative and in need of repair.

[0461] At block 2616, the RMC sends either the creating new file requestor the range lock acquisition request to the primary metaserver. If therequested operation on the primary metaserver fails (but the primarymetaserver is available), the FAIL path is taken and the RMC fails theoperation request; if the primary metaserver is unavailable, anothermetaserver is chosen as a primary and the operation is retried. In thealternative, if the access to the primary metaserver is successful, theSUCCESS path is taken and the method moves to block 2618.

[0462] At block 2618, the RMC sends either the creating new filerequests or the lock acquisition requests to all other metaservers. Itis expected that the operation will succeed on those other metaservers;an unexpected failure (other than the metadata server just beingunavailable) is usually an indication of inconsistency among themetadata servers.

[0463] At block 2620, based on the successful accesses to a metaserverin either block 2610, 2614 or 2618, the RMC saves a primary metaserverstatus in accordance with the metaserver or metaservers thatsuccessfully handled the access operation.

[0464] At block 2622, the RMC saves the states of the availablemetaservers and responds to the requested operation.

[0465] At block 2624, the RMC saves states information indicating whichmetaservers successfully handled the access operation. Preferably, theseare the only metaservers to which subsequent operations for thismetafile will be sent. For some operations, this step 2624 may beskipped. The method then ends in block 2626.

[0466] The foregoing description, for purposes of explanation, has beendescribed with reference to specific embodiments. However, theillustrative discussions above are not intended to be exhaustive or tolimit the invention to the precise forms disclosed. Many modificationsand variations are possible in view of the above teachings. Theembodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method of processing a user request to performan operation on a user file, comprising: receiving one or more userrequests to perform a specified operation on a specified user file;selecting, from among a group of rules, a rule applicable to thespecified user file; each rule in the group of rules including at leastone parameter specifying, for files to which the rule is applicable, howto distribute storage of each such file over the file servers in a groupof file servers; and performing the specified operation in accordancewith the selected rule, including, when the specified operation changesthe size of the specified user file, sending commands to a plurality ofthe file servers so as to continue to distribute storage of thespecified user file in accordance with the selected rule.
 2. The methodof claim 1, the performing step including updating storage of the userfile over the group of file servers in accordance with the selectedrule.
 3. A method of processing a user request to perform an operationon a user file, comprising: receiving one or more user requests toperform a specified operation on a specified user file; selecting, fromamong a group of rules, a rule applicable to the specified user file;each rule in the group of rules including a striping parameterspecifying, for files to which the rule is applicable, a maximum size offile portions into which the files are divided, and a mirroringparameter specifying, for files to which the rule is applicable, anumber of instances of each file portion are to be stored on fileservers in a group of file servers; and performing the specifiedoperation in accordance with the selected rule, including: when thespecified operation increases the size of the specified user file bymore than the maximum file portion size specified by the selected rule,adding one or more file portions to the user file, including creatingand storing on the group of file servers the specified number ofinstances of each added file portion, each created file portion instancehaving a size no greater than the maximum file portion size specified bythe selected rule.
 4. The method of claim 3, including storing in ametadata file, separate from the file portion instances in which thespecified user file is stored, data identifying a respective file serveron which is stored each file portion instance of the specified userfile.
 5. The method of claim 4, including when the specified operationaccesses data previously stored within the specified user file,generating an index value corresponding to a location to be accessedwithin the specified user file divided by the maximum file portion sizespecified by the selected rule, accessing the metafile using thegenerated index value to identify the file server on which is stored afile portion instance containing the location to be accessed within thespecified user file, and then accessing said file portion instance onthe identified file server.
 6. The method of claim 3, including when thespecified operation accesses data previously stored within the specifieduser file, generating an index value corresponding to a location to beaccessed within the specified user file divided by the maximum fileportion size specified by the selected rule, and then accessing a fileportion instance of the specified user file corresponding to thegenerated index value.
 7. The method of claim 3, including when thespecified operation accesses data previously stored within the specifieduser file, generating a set of index values in accordance with theselected rule, the set of index values corresponding to a range oflocations to be accessed within the specified user file, and thenaccessing a set of file portion instances of the specified user filecorresponding to the generated set of index values.
 8. The method ofclaim 3, wherein each rule in the group of rules has an associated setof files to which the rule is applicable, wherein the set of files for aparticular rule in the group of rules is specified by a specificationselected from the group consisting essentially of A) files having a filepath matching a specified file path, B) files of a specified file type.9. The method of claim 3, wherein the rules in the group of rules areordered, and each rule in the group of rules has an associated range offiles to which the rule is applicable, wherein the range of files for aparticular rule in the group of rules is specified by a specificationselected from the group consisting essentially of A) files not fallingwithin the range of any higher order rule and having a file pathmatching a specified file path, B) files not falling within the range ofany higher order rule and comprising a file of a specified file type, C)files not falling within the range of any higher order rule and having aspecified owner.
 10. The method of claim 3, wherein each rule in thegroup of rules includes a caching parameter specifying, for files towhich the rule is applicable, a maximum cache size for caching portionsof any particular file to which the rule is applicable; and when cachingdata retrieved from or written to the specified user file, caching nomore data than specified by the caching parameter specified by theselected rule.
 11. The method of claim 3, wherein each rule in the groupof rules includes a read ahead caching parameter specifying whether readahead caching is enabled for files to which the rule is applicable; andwhen retrieved data from the specified user file, performing read aheadcaching only if read ahead caching is enabled by the read ahead cachingparameter specified by the selected rule.
 12. The method of claim 3,including identifying a subset of the specified user file on which toperform the specified operation; and wherein performing the specifiedoperation includes: accessing a metafile that stores informationidentifying a set of file servers for storing the respective fileportion instances of the specified user file; and when the specifiedoperation accesses data previously stored within the specified userfile, determining, based on the information stored in the metafile, asubset of the file servers that store an instance of each of theportions of the user file that together comprise the identified subsetof the specified user file.
 13. The method of claim 12, whereinaccessing a metafile comprises accessing a cached information from themetafile.
 14. A file switch for use in a computer network having a groupof file servers and a plurality of client computers, the file switchcomprising: at least one processing unit for executing computerprograms; at least one port for exchanging information with the fileservers and client computers, the information exchanged includinginformation concerning a specified user file; a file aggregation moduleincluding one or more computer programs, the computer programs includinginstructions for: receiving one or more user requests to perform aspecified operation on a specified user file; selecting, from among agroup of rules, a rule applicable to the specified user file; each rulein the group of rules including at least one parameter specifying, forfiles to which the rule is applicable, how to distribute storage of eachsuch file over the file servers in a group of file servers; andperforming the specified operation in accordance with the selected rule,including, when the specified operation changes the size of thespecified user file, sending commands to a plurality of the file serversso as to continue to distribute storage of the specified user file inaccordance with the selected rule.
 15. The file switch of claim 14, theinstructions for performing including instructions for updating storageof the user file over the group of file servers in accordance with theselected rule.
 16. A file switch for use in a computer network having agroup of file servers and a plurality of client computers, the fileswitch comprising: at least one processing unit for executing computerprograms; at least one port for exchanging information with the fileservers and client computers, the information exchanged includinginformation concerning a specified user file; a file aggregation moduleincluding one or more computer programs, the computer programs includinginstructions for: receiving one or more user requests to perform aspecified operation on a specified user file; selecting, from among agroup of rules, a rule applicable to the specified user file; each rulein the group of rules including a striping parameter specifying, forfiles to which the rule is applicable, a maximum size of file portionsinto which the files are divided, and a mirroring parameter specifying,for files to which the rule is applicable, a number of instances of eachfile portion are to be stored on file servers in the group of fileservers; and performing the specified operation in accordance with theselected rule, including: when the specified operation increases thesize of the specified user file by more than the maximum file portionsize specified by the selected rule, adding one or more file portions tothe user file, including creating and storing on the group of fileservers the specified number of instances of each added file portion,each created file portion instance having a size no greater than themaximum file portion size specified by the selected rule.
 17. The fileswitch of claim 16, wherein the one or more computer programs of thefile aggregation module include instructions for: storing in a metadatafile, separate from the file portion instances in which the specifieduser file is stored, data identifying a respective file server on whichis stored each file portion instance of the specified user file.
 18. Thefile switch of claim 17, wherein the one or more computer programs ofthe file aggregation module include instructions for: when the specifiedoperation accesses data previously stored within the specified userfile, generating an index value corresponding to a location to beaccessed within the specified user file divided by the maximum fileportion size specified by the selected rule, accessing the metafileusing the generated index value to identify the file server on which isstored a file portion instance containing the location to be accessedwithin the specified user file, and then accessing said file portioninstance on the identified file server.
 19. The file switch of claim 16,wherein the one or more computer programs of the file aggregation moduleinclude instructions for: when the specified operation accesses datapreviously stored within the specified user file, generating an indexvalue corresponding to a location to be accessed within the specifieduser file divided by the maximum file portion size specified by theselected rule, and then accessing a file portion instance of thespecified user file corresponding to the generated index value.
 20. Thefile switch of claim 16, wherein the one or more computer programs ofthe file aggregation module include instructions for: when the specifiedoperation accesses data previously stored within the specified userfile, generating a set of index values in accordance with the selectedrule, the set of index value corresponding to a range of locations to beaccessed within the specified user file, and then accessing a set offile portion instances of the specified user file corresponding to thegenerated set of index values.
 21. The file switch of claim 16, whereineach rule in the group of rules has an associated set of files to whichthe rule is applicable, wherein the set of files for a particular rulein the group of rules is specified by a specification selected from thegroup consisting essentially of A) files having a file path matching aspecified file path, B) files of a specified file type.
 22. The fileswitch of claim 16, wherein the rules in the group of rules are ordered,and each rule in the group of rules has an associated range of files towhich the rule is applicable, wherein the range of files for aparticular rule in the group of rules is specified by a specificationselected from the group consisting essentially of A) files not fallingwithin the range of any higher order rule and having a file pathmatching a specified file path, B) files not falling within the range ofany higher order rule and comprising a file of a specified file type, C)files not falling within the range of any higher order rule and having aspecified owner.
 23. The file switch of claim 16, wherein each rule inthe group of rules includes a caching parameter specifying, for files towhich the rule is applicable, a maximum cache size for caching portionsof any particular file to which the rule is applicable; and the one ormore computer programs of the file aggregation module includeinstructions for caching no more data than specified by the cachingparameter specified by the selected rule when caching data retrievedfrom or written to the specified user file.
 24. The file switch of claim16, wherein each rule in the group of rules includes a read aheadcaching parameter specifying whether read ahead caching is enabled forfiles to which the rule is applicable; and the one or more computerprograms of the file aggregation module include instructions forperforming read ahead caching, when retrieving data from the specifieduser file, only if read ahead caching is enabled by the read aheadcaching parameter specified by the selected rule.
 25. The file switch ofclaim 16, wherein the one or more computer programs of the fileaggregation module include instructions for including identifying asubset of the specified user file on which to perform the specifiedoperation; and the instructions for performing the specified operationinclude instruction for: accessing a metafile that stores informationidentifying a set of file servers for storing the respective fileportion instances of the specified user file; and when the specifiedoperation accesses data previously stored within the specified userfile, determining, based on the information stored in the metafile, asubset of the file servers that store an instance of each of theportions of the user file that together comprise the subset of thespecified user file.
 26. The file switch of claim 25, wherein theinstructions for accessing a metafile comprise instructions foraccessing cached information from the metafile.
 27. A system for use ina computer network having a plurality of client computers, the systemcomprising: a group of file servers; a file switch, the file switchincluding: at least one port for exchanging information with the fileservers and client computers, the information exchanged includinginformation concerning a specified user file; means for receiving one ormore user requests to perform a specified transaction on a specifieduser file; means for selecting, from among a group of rules, a ruleapplicable to the specified user file; each rule in the group of rulesincluding at least one parameter specifying, for files to which the ruleis applicable, how to distribute storage of each such file over the fileservers in a group of file servers; and means for performing thespecified operation in accordance with the selected rule, including,when the specified operation changes the size of the specified userfile, sending commands to a plurality of the file servers so as tocontinue to distribute storage of the specified user file in accordancewith the selected rule.
 28. The system of claim 27, the means forperforming including means for updating storage of the user file overthe group of file servers in accordance with the selected rule.
 29. Asystem for use in a computer network having a plurality of clientcomputers, the file switch comprising: a group of file servers; a fileswitch, the file switch including: at least one port for exchanginginformation with the file servers and client computers, the informationexchanged including information concerning a specified user file; meansfor receiving one or more user requests to perform a specifiedtransaction on a specified user file; means for selecting, from among agroup of rules, a rule applicable to the specified user file; each rulein the group of rules including a striping parameter specifying, forfiles to which the rule is applicable, a maximum size of file portionsinto which the files are divided, and a mirroring parameter specifying,for files to which the rule is applicable, a number of instances of eachfile portion are to be stored on file servers in a group of fileservers; and means performing the specified operation in accordance withthe selected rule, including, when the specified operation increases thesize of the specified user file by more than the maximum file portionsize specified by the selected rule, adding one or more file portions tothe user file, including creating and storing on the group of fileservers the specified number of instances of each added file portion,each created file portion instance having a size no greater than themaximum file portion size specified by the selected rule.
 30. The systemof claim 29, including means for storing in a metadata file, separatefrom the file portion instances in which the specified user file isstored, data identifying a respective file server on which is storedeach file portion instance of the specified user file.
 31. The system ofclaim 29, wherein the means for performing includes means, operativewhen the specified operation accesses data previously stored within thespecified user file, for generating an index value corresponding to alocation to be accessed within the specified user file divided by themaximum file portion size specified by the selected rule, accessing themetafile using the generated index value to identify the file server onwhich is stored a file portion instance containing the location to beaccessed within the specified user file, and then accessing said fileportion instance on the identified file server.
 32. The system of claim29, wherein the means for performing includes means, operative when thespecified operation accesses data previously stored within the specifieduser file, for generating an index value corresponding to a location tobe accessed within the specified user file divided by the maximum fileportion size specified by the selected rule, and then accessing a fileportion instance of the specified user file corresponding to thegenerated index value.
 33. The system of claim 29, wherein the means forperforming includes means, operative when the specified operationaccesses data previously stored within the specified user file, forgenerating a set of index values in accordance with the selected rule,the set of index values corresponding to a range of locations to beaccessed within the specified user file, and then accessing a set offile portion instances of the specified user file corresponding to thegenerated set of index values.
 34. The system of claim 29, wherein eachrule in the group of rules has an associated set of files to which therule is applicable, wherein the set of files for a particular rule inthe group of rules is specified by a specification selected from thegroup consisting essentially of A) files having a file path matching aspecified file path, B) files of a specified file type.
 35. The systemof claim 29, wherein the rules in the group of rules are ordered, andeach rule in the group of rules has an associated range of files towhich the rule is applicable, wherein the range of files for aparticular rule in the group of rules is specified by a specificationselected from the group consisting essentially of A) files not fallingwithin the range of any higher order rule and having a file pathmatching a specified file path, B) files not falling within the range ofany higher order rule and comprising a file of a specified file type, C)files not falling within the range of any higher order rule and having aspecified owner.
 36. The system of claim 29, wherein each rule in thegroup of rules includes a caching parameter specifying, for files towhich the rule is applicable, a maximum cache size for caching portionsof any particular file to which the rule is applicable; the file switchincluding caching means for caching data retrieved from or written tothe specified user file, the caching means including means for cachingno more data than specified by the caching parameter specified by theselected rule.
 37. The system of claim 29, wherein each rule in thegroup of rules includes a read ahead caching parameter specifyingwhether read ahead caching is enabled for files to which the rule isapplicable; and when retrieved data from the specified user file,performing read ahead caching only if read ahead caching is enabled bythe read ahead caching parameter specified by the selected rule.
 38. Thesystem of claim 29, the file switch including means for identifying asubset of the specified user file on which to perform the specifiedoperation; wherein the means for performing the specified operationincludes: means for accessing a metafile that stores informationidentifying a set of file servers for storing the respective fileportion instances of the specified user file; and means, operative whenthe specified operation accesses data previously stored within thespecified user file, for determining, based on the information stored inthe metafile, a subset of the file servers that store an instance ofeach of the portions of the user file that together comprise theidentified subset of the specified user file.
 39. The system of claim29, wherein the means for accessing a metafile comprises means foraccessing a cached information from the metafile.
 40. A system for usein a computer network having a plurality of client computers, the systemcomprising: a group of file servers; a file switch, the file switchincluding: at least one port for exchanging information with the fileservers and client computers, the information exchanged includinginformation concerning a specified user file; at least one processingunit for executing computer programs; a file aggregation moduleincluding one or more computer programs, the computer programs includinginstructions for: receiving one or more user requests to perform aspecified operation on a specified user file; selecting, from among agroup of rules, a rule applicable to the specified user file; each rulein the group of rules including at least one parameter specifying, forfiles to which the rule is applicable, how to distribute storage of eachsuch file over the file servers in a group of file servers; andperforming the specified operation in accordance with the selected rule,including, when the specified operation changes the size of thespecified user file, sending commands to a plurality of the file serversso as to continue to distribute storage of the specified user file inaccordance with the selected rule.
 41. The system of claim 40, theinstructions for performing including instructions for updating storageof the user file over the group of file servers in accordance with theselected rule.
 42. A system for use in a computer network having aplurality of client computers, the system comprising: a group of fileservers; a file switch, the file switch including: at least one port forexchanging information with the file servers and client computers, theinformation exchanged including information concerning a specified userfile; at least one processing unit for executing computer programs; afile aggregation module including one or more computer programs, thecomputer programs including instructions for: receiving one or more userrequests to perform a specified operation on a specified user file;selecting, from among a group of rules, a rule applicable to thespecified user file; each rule in the group of rules including astriping parameter specifying, for files to which the rule isapplicable, a maximum size of file portions into which the files aredivided, and a mirroring parameter specifying, for files to which therule is applicable, a number of instances of each file portion are to bestored on file servers in the group of file servers; and performing thespecified operation in accordance with the selected rule, including:when the specified operation increases the size of the specified userfile by more than the maximum file portion size specified by theselected rule, adding one or more file portions to the user file,including creating and storing on the group of file servers thespecified number of instances of each added file portion, each createdfile portion instance having a size no greater than the maximum fileportion size specified by the selected rule.
 43. The system of claim 42,wherein the one or more computer programs of the file aggregation moduleinclude instructions for: storing in a metadata file, separate from thefile portion instances in which the specified user file is stored, dataidentifying a respective file server on which is stored each fileportion instance of the specified user file.
 44. The system of claim 43,wherein the one or more computer programs of the file aggregation moduleinclude instructions for: when the specified operation accesses datapreviously stored within the specified user file, generating an indexvalue corresponding to a location to be accessed within the specifieduser file divided by the maximum file portion size specified by theselected rule, accessing the metafile using the generated index value toidentify the file server on which is stored a file portion instancecontaining the location to be accessed within the specified user file,and then accessing said file portion instance on the identified fileserver.
 45. The system of claim 42, wherein the one or more computerprograms of the file aggregation module include instructions for: whenthe specified operation accesses data previously stored within thespecified user file, generating an index value corresponding to alocation to be accessed within the specified user file divided by themaximum file portion size specified by the selected rule, and thenaccessing a file portion instance of the specified user filecorresponding to the generated index value.
 46. The system of claim 42,wherein the one or more computer programs of the file aggregation moduleinclude instructions for: when the specified operation accesses datapreviously stored within the specified user file, generating a set ofindex values in accordance with the selected rule, the set of indexvalue corresponding to a range of locations to be accessed within thespecified user file, and then accessing a set of file portion instancesof the specified user file corresponding to the generated set of indexvalues.
 47. The system of claim 42, wherein each rule in the group ofrules has an associated set of files to which the rule is applicable,wherein the set of files for a particular rule in the group of rules isspecified by a specification selected from the group consistingessentially of A) files having a file path matching a specified filepath, B) files of a specified file type.
 48. The system of claim 42,wherein the rules in the group of rules are ordered, and each rule inthe group of rules has an associated range of files to which the rule isapplicable, wherein the range of files for a particular rule in thegroup of rules is specified by a specification selected from the groupconsisting essentially of A) files not falling within the range of anyhigher order rule and having a file path matching a specified file path,B) files not falling within the range of any higher order rule andcomprising a file of a specified file type, C) files not falling withinthe range of any higher order rule and having a specified owner.
 49. Thesystem of claim 42, wherein each rule in the group of rules includes acaching parameter specifying, for files to which the rule is applicable,a maximum cache size for caching portions of any particular file towhich the rule is applicable; and the one or more computer programs ofthe file aggregation module include instructions for caching no moredata than specified by the caching parameter specified by the selectedrule when caching data retrieved from or written to the specified userfile.
 50. The system of claim 42, wherein each rule in the group ofrules includes a read ahead caching parameter specifying whether readahead caching is enabled for files to which the rule is applicable; andthe one or more computer programs of the file aggregation module includeinstructions for performing read ahead caching, when retrieving datafrom the specified user file, only if read ahead caching is enabled bythe read ahead caching parameter specified by the selected rule.
 51. Thesystem of claim 42, wherein the one or more computer programs of thefile aggregation module include instructions for including identifying asubset of the specified user file on which to perform the specifiedoperation; and the instructions for performing the specified operationinclude instruction for: accessing a metafile that stores informationidentifying a set of file servers for storing the respective fileportion instances of the specified user file; and when the specifiedoperation accesses data previously stored within the specified userfile, determining, based on the information stored in the metafile, asubset of the file servers that store an instance of each of theportions of the user file that together comprise the subset of thespecified user file.
 52. The system of claim 51, wherein theinstructions for accessing a metafile comprise instructions foraccessing cached information from the metafile.